Company history

Kamioka is said to have been opened for copper in the Wadō era of 708, and gold, silver, lead and zinc were dug there through the Nara, Sengoku and Edo periods — a history Mitsui Kinzoku still claims as its own origin. The company’s corporate story, though, begins in 1874, when Mitsui-gumi took over the Jabaradaira adit from the Meiji government and entered non-ferrous mining under a policy of state-led industrialisation, with government demand and the zaibatsu’s own trading channels behind it.

Kamioka turned out to be the largest zinc producer in Japan and a lead-zinc mine of world rank, spoken of alongside Sullivan in Canada and Broken Hill in Australia. In 1892 Mitsui Mining was formed as a partnership to run the metal mines and the collieries together, and in 1913 a zinc smelter at Omuta closed the loop: concentrate mined at Kamioka was refined in-house all the way to electrolytic zinc metal. That upstream-to-midstream integration — dig it, smelt it, sell the metal — was the business model the post-war company would inherit intact.

The break-up of the zaibatsu separated the metals division from Mitsui Mining on 1 May 1950 under the Enterprise Reconstruction and Reorganisation Act. Barred at first from using the Mitsui name, it started as Kamioka Mining with capital of $1.7M (¥600m), listed in Tokyo within months of its founding, doubled its capital to ¥1.2 billion in October 1951, opened a Tokyo research laboratory that December, and in December 1952 took the name Mitsui Mining & Smelting. What had been cut loose was not a wind-down case but a business that stood on its own: a mine with twelve centuries of workings behind it and the Miike, Hikoshima and Hibi smelters attached.

The two decades that followed were spent widening that base. Smelting plant was modernised in quick succession from 1953, and in the 1960s the company pushed downstream — rolled-copper and die-casting divisions in 1962, the absorption of Oji Kinzoku and Showa Die Casting the same year — and outward, taking a stake in the Huanzala mine in Peru in 1964 and setting up Hachinohe Smelting in 1967, which put zinc refining on three sites: Kamioka, Hikoshima and Hachinohe. Capital rose from ¥2.4 billion in 1954 to ¥10.8 billion by 1965. Yet the deeper the integration went, the more plainly the whole structure still rested on one mine and the zinc price.

In 1972 the company lost its appeal in the Itai-itai disease litigation. Cadmium in effluent from Kamioka had, over decades, poisoned the Jinzu river basin in Toyama; Mitsui Kinzoku chose not to appeal further, accepted compensation, and signed on to annual site inspections by the plaintiffs and a soil-restoration programme that would run for thirty-three years. The dividend was suspended for the year ended March 1973. From then on, environmental cost was a permanent constraint on the founding business rather than an episode in it.

The Nixon shock and a rising yen took away what price competitiveness Kamioka had left, and because its mine was so much bigger than its domestic rivals’, Mitsui Kinzoku was slower than any of them to admit the ore was finished. Temporary layoffs began in 1978; a restructuring plan with job cuts followed in 1981; the Plaza Accord of 1985 made it worse, and a further round of voluntary redundancies came in 1986, the year Kamioka was hived off into a wholly owned subsidiary so that it could be shrunk on its own schedule. Lead smelting from ore stopped in 1994, a plan cutting another fifth of the workforce came in 1995, and in June 2001 ore extraction ceased altogether — closing roughly 130 years of continuous mining.

What made this a transition rather than a liquidation was that the replacement was built in the same years. Oak-Mitsui was set up in the United States in 1976 to make copper foil for printed circuit boards; the Ageo foil plant followed in 1980; a subsidiary for semiconductor TAB tape was founded in 1989. The fit was technical, not opportunistic — the electrolysis and surface chemistry that produced refined metal were most of what fine copper foil required. Automotive parts grew alongside, with GECOM in the United States from 1987 and overseas catalyst production from 1995, and in 2001 the MAP500 mid-term plan finally said out loud that electronic materials, not ore, were the core.

The 2000s were spent rearranging what was left. Oi Seisakusho was taken over fully in 2003 to secure the automotive-parts supply chain; Pallca in Peru started up in 2006; the rolled-copper business was merged into a joint venture with Sumitomo Metal Mining in 2010, the same year the company took a share of Caserones in Chile. The portfolio grew more selective, but it stayed two-layered — and the metal layer still governed the results. In the year ended March 2009 the collapse in metal prices produced a net loss of $718.5M (¥67bn), eight years after the company had declared itself an electronics materials firm.

It was the second half of the 2010s that finally shifted the balance. As printed circuit boards grew finer and high-frequency designs spread, high-value grades such as MicroThin and VSP carried the electronic materials segment, and Mitsui Kinzoku came to hold roughly 90% of the world market in ultra-thin copper foil — the layer on which semiconductor package substrates are built. Exhaust catalysts, sold into Japan, Europe, North America and India, became a second international pillar, and door latches and hinges from GECOM a third.

Record profits followed the metal-price surge in the year to March 2022, and again as AI server demand pulled at substrate materials. Kamioka Mining, converted to recycling smelting after the mine closed, posted ¥43.5 billion of sales in the year to March 2024. The concentration went further in the 2025 financial year, when the company took a $127M (¥19bn) special loss to let go of Mitsui Kinzoku ACT, the automotive-parts arm whose outlook the shift to EVs had narrowed, and put functional materials at the centre of its new mid-term plan. The logic that closed Kamioka is still running.