Mitsui Main Building

The Mitsui Main Building (三井本館, Mitsui Honkan) is a landmark seven-storey, steel-framed reinforced concrete building located at Nihonbashi-Muromachi 2-chome, Chuo Ward, Tokyo. It was completed in 1929 as the new headquarters of the Mitsui Zaibatsu — Japan's largest, wealthiest, and oldest financial-industrial conglomerate, tracing its origins to the Edo-period merchant house founded by Mitsui Takatoshi in 1673.

The previous Mitsui headquarters on this site, built in 1902, suffered catastrophic damage in the Great Kanto Earthquake of September 1, 1923. Under the direction of Mitsui Gomei Kaisha president Hachiroemon Takamine Mitsui and managing director Takuma Dan, the decision was made to construct an earthquake-resistant building that would serve as a model for Tokyo's reconstruction. The American architectural firm Trowbridge & Livingston, renowned for designing the St. Regis Hotel and J.P. Morgan Building in New York, was commissioned. The result was a monumental Beaux-Arts neoclassical structure embodying principles of "magnificence, elegance, and simplicity," featuring Corinthian columns along its facade, state-of-the-art earthquake resistance, central air conditioning, and advanced fire suppression systems.

During the 1930s and 1940s, the Mitsui Main Building served as the command center of an industrial empire that grew to encompass approximately 270 companies by war's end. The Mitsui Zaibatsu's core enterprises — Mitsui Bank (三井銀行), Mitsui Bussan/Trading (三井物産), Mitsui Mining (三井鉱山), and Mitsui Chemical (三井化学) — were deeply integrated into Japan's wartime economic mobilization. Following the Manchurian Incident of 1931 and escalation into full-scale war with China in 1937, Mitsui aggressively expanded into heavy chemical industries centered on military procurement, with total direct investments exceeding 600 million yen.

As Japan's wartime control economy intensified, Mitsui played a pivotal civilian role in the national mobilization system. The zaibatsu expanded operations into occupied Manchuria, China, and Southeast Asia at the military's request, establishing resource extraction networks for coal, metals, and strategic materials vital to the war effort. In 1943, under government pressure for financial consolidation to support military procurement, Mitsui Bank merged with Dai-Ichi Bank to form the Teikoku Bank (帝国銀行). That same year, the "Shanxi Incident" (山西事件) saw military authorities target Mitsui Bussan for alleged control violations, forcing leadership resignations and intensifying public criticism of zaibatsu power. In 1944, Mitsui reorganized into a formal holding company structure overseeing ten direct affiliates and twelve semi-affiliates. Throughout the war, Mitsui employed forced laborers including Allied prisoners of war — some of whom were maimed by Mitsui employees — in mining and industrial operations.

Following Japan's surrender on August 15, 1945, the Supreme Commander for the Allied Powers (SCAP/GHQ) designated Mitsui among the zaibatsu targeted for dissolution under the Deconcentration Policy. The Mitsui family's holding company was dissolved, its assets frozen, and its constituent companies broken apart. Mitsui Bussan, one of the world's largest trading companies, was completely dissolved. The Mitsui family members were purged from leadership positions and had 60% of personal income confiscated, with massive unpaid inheritance taxes exceeding 40 million yen across the eleven Mitsui families. The zaibatsu gradually reconstituted itself in the 1950s as a horizontal keiretsu coalition of independent companies.

The Mitsui Main Building itself survived the Tokyo firebombing raids of 1945 largely intact. In 1998, it was designated a National Important Cultural Property (国指定重要文化財) as an outstanding example of early Showa-era Western-style architecture and a pioneering design for multi-tenant office buildings in Japan. Today, the building houses the headquarters of Mitsui Fudosan (三井不動産), branches of Sumitomo Mitsui Banking Corporation, and the Mitsui Memorial Museum (三井記念美術館). The adjacent 39-storey Nihonbashi Mitsui Tower, completed in 2005, was carefully designed to complement the historic building while providing modern office space.