Company history

The oldest of the group’s ancestors was not a zaibatsu company at all. Osaka Insurance, founded in 1893 under Masuda Nobuyuki, president of Osaka Seido, was a Kansai regional underwriter writing both fire and marine. It joined the Osaka Shosen shipping group in 1916 and became Osaka Marine & Fire; carried by the shipping boom of the First World War, its premium income ranked second in the industry the following year. A second line began in 1917, when Yamashita Saburo of Yamashita Kisen proposed Fuso Marine and gathered Shibusawa Eiichi and Sumitomo Kichizaemon among its promoters — a start capitalized at ¥10 million, exceptional for the day. Fuso added fire cover in 1920, took Sumitomo Goshi as its largest shareholder in 1927, and in 1940 renamed itself Sumitomo Marine & Fire. A merchant-city insurer and a shipping-and-zaibatsu insurer, opposite in character, were now within reach of each other.

The Mitsui line began in October 1918, when Odagaki Jujiro, a managing director of Mitsui & Co., assembled leading businessmen to found Taisho Marine & Fire with ¥500,000 in capital. It registered in Tokyo but ran its sales headquarters from Kobe, where Mitsui Bussan’s shipping department sat and where Japan’s foreign trade was then centred. Deliberately, the name said nothing about Mitsui: the house’s influence was large, the future of non-life insurance was not yet settled in the public mind, and no one wanted to hang the Mitsui sign on an uncertain trade. A blue-chip promoter list under a studiously bland name — the caution is the Mitsui temperament itself.

The Great Kanto Earthquake burned out the Mitsui No. 2 building that housed the head office, and Taisho paid consolation money on earthquake losses it bore no contractual duty to cover, following the government’s arbitration. On top of marine, fire and transit it took licences for motor and personal accident in 1928, aviation in 1937 and business-interruption cover in 1938; by 1937 it stood third among 47 insurers in premium income, and by 1940 second in marine. The name, though, would not change until April 1991 — seventy-three years — with wartime criticism of the zaibatsu and postwar dissolution standing in between.

In March 1944, inside the wartime rationalization of the non-life industry, Osaka Marine and the old Sumitomo Marine merged at the government’s request into Osaka Sumitomo Marine & Fire, third-largest in the industry and clearly first among Kansai-based insurers. It took the name Sumitomo Marine & Fire again in July 1954. The Sumitomo house, in other words, was not assembled by ambition but by state order, sewing a merchant-city underwriter to a shipping-and-zaibatsu one.

After the war both companies faced a weak marine market and had to build domestic lines and rebuild overseas business at the same time — and they chose opposite routes. The Mitsui side, still trading as Taisho, opened new classes ahead of rivals: forestry, theft, racehorse, credit, storm and flood, plate glass. In 1961 it led the underwriting of reactor-assembly cover at Tokai-mura, stepping into nuclear risk, and in 1964 it issued London Depositary Receipts — the first Japanese company to price new equity in the London market. Widening the product list at home and reaching capital markets abroad became the two axes of its postwar growth.

Sumitomo Marine bet instead on operating efficiency and savings-type products. Its first, long-term comprehensive insurance, launched in 1969, drove the balance-sheet growth that followed; the women’s savings policy it developed alone in 1984 sold explosively and won a Nikkei product-of-the-year award. It organized its agents nationally in 1968, led the industry on operating balance ratio in the 1965 accounts, and opened fifty branches at a stroke across 1978–79. Two top-ranked insurers, one a product pioneer and the other an efficiency machine — the contrast that would later make them complementary merger partners was laid down here.

In April 1991, on its 73rd anniversary and after long consultation inside and outside the firm, Taisho Marine & Fire finally became Mitsui Marine & Fire — nearly half a century after the dissolution of the zaibatsu, and the first time a company spun out of Mitsui & Co. put the house name on its own door. Sumitomo Marine was by then running its own long-range plan. What changed the stakes for both was the financial Big Bang and the deregulation of non-life insurance in the late 1990s, which turned consolidation pressure on the industry up sharply.

In October 2001 the industry’s number three and number four combined: Mitsui Sumitomo Insurance, a near-equal merger of a Mitsui house strong in new lines and overseas capital markets with a Sumitomo house strong in efficiency and savings products. It formed one of the three mega non-life groups beside Tokio Marine Nichido and Sompo Japan, though unifying product ranges and distribution took years. Wartime consolidation in 1944, the Mitsui renaming in 1991, the Mitsui–Sumitomo union in 2001 — half a century of intermittent mergers had arrived at a single company.

Then came the move that defined the group. In 2007 Mitsui Sumitomo’s board resolved to create a holding company by sole share transfer, and in April 2008 Mitsui Sumitomo Insurance Group Holdings listed simultaneously in Tokyo, Osaka and Nagoya. The point was less to streamline one company than to have the receiving platform ready before the industry finished converging. Two years later it was used: an April 2010 share exchange brought in Aioi Insurance, whose largest shareholder was Toyota, and Nissay Dowa, whose largest shareholder was Nippon Life; the holding company was renamed MS&AD Insurance Group Holdings, and in October the two newcomers merged into Aioi Nissay Dowa. The combined net premiums were the largest in Japan.

MS&AD did not merge its core insurers. Mitsui Sumitomo (MS) and Aioi Nissay Dowa (AD) kept separate brands and separate distribution, and in September 2013 the group signed a functional-reorganization agreement that integrated only products, claims service and systems, targeting $512.3M (¥50bn) of expense reduction. Ten years on it had beaten that target and cut roughly $71.2M (¥10bn) more through product integration, while the two companies’ combined revenue grew from about ¥2.4 trillion in the year to March 2013 to some ¥3.0 trillion by March 2025. But duplicated head-office functions and double investment in claims infrastructure survived, analysts asked about a merger year after year, and management kept it as an option it was in no hurry to exercise — partly because Aioi’s Toyota dealer channel and MS’s agency network were different animals, and forcing them together risked chaos in the field.

Overseas, the same disposition played out on a larger stage. In February 2016 the group bought Amlin plc, a leading Lloyd’s specialty underwriter, for about $5.8B (¥630bn) — then one of the largest overseas deals ever done by a Japanese non-life insurer. Amlin missed plan for years, absorbed catastrophe losses and let underwriting discipline slip, and weighed on group adjusted profit for a long time. MS&AD did not leave: in October 2020 it sold the plc and restructured the business, keeping only the MS Amlin brand, replacing management, exiting unprofitable lines and re-imposing underwriting discipline. A 2022 investment in Transverse Insurance Group, a US MGA platform, marked the switch from single large acquisitions to North American bolt-ons, and the overseas book finally turned reliably profitable in the early 2020s.

The forcing event came from outside. In 2024 the industry-wide premium-fixing scandal brought MS&AD a $8.6M (¥1bn) surcharge from the Fair Trade Commission, and with the Big Motor and data-leak affairs it drove tighter rules on the agency model itself. Because cross-shareholdings were identified as one root of the practice of matching rates with rivals, insurers were pushed to zero out policy shareholdings by March 2030: MS&AD cut a $23.8B (¥3.6tn) portfolio at March 2024 to $15.4B (¥2.3tn) by September 2025, selling roughly 42% in two years and recycling the proceeds into growth investment and shareholder returns. Net profit hit a record $4.6B (¥692bn) in the year to March 2025, with a thirteenth straight dividend increase. Then, in November 2025, the group put about $4.0B (¥600bn) into the founding family of the North American specialty insurer W. R. Berkley to widen underwriting cooperation abroad — and announced that MS and AD would finally merge in April 2027, aiming at $1.0B (¥150bn) of cost savings and ¥700 billion of adjusted profit in FY2030.