Former 4th Division Headquarters (MIRAIZA OSAKA-JO)

The former 4th Division Headquarters (旧第四師団司令部庁舎, Kyū Dai-yon Shidan Shireibu Chōsha) is a landmark Romanesque-revival military building standing on the Honmaru of Osaka Castle in Chūō-ku, Osaka. It is one of the most historically layered structures in modern Osaka, having served continuously for nearly a century under radically different masters — the Imperial Japanese Army, the US occupation, Japanese police forces, a municipal museum, and now a commercial entertainment complex.

Construction and Architecture (1928–1931)

In 1928, the citizens of Osaka launched a fundraising campaign to commemorate the enthronement of Emperor Hirohito (Shōwa). The 1.5 million yen collected funded three concurrent projects: reconstruction of the Osaka Castle main tower (天守閣), creation of Osaka Castle Park, and construction of a new headquarters building for the 4th Division of the Imperial Japanese Army. The building was designed in a dignified Romanesque revival style — a deliberate choice that harmonized with the castle setting while conveying military authority. It features a symmetrical plan with a central entrance flanked by round turrets (タレット), long lateral wings, brown scratch-tile (スクラッチタイル) cladding, Romanesque arched decorative motifs along the roofline, and a three-story reinforced concrete structure. Construction was completed in 1931.

The 4th Division: From Osaka Garrison to Pacific War

The 4th Division (第四師団) traced its origins to 1871 (Meiji 4), when the Osaka Garrison (大阪鎮台) was established. It was one of the four regional garrisons of the new Imperial Japanese Army at the time (increased to six in 1873) and was reorganized as the 4th Division in 1888. The division's history spans all of Japan's modern wars: it served as a reserve division in the First Sino-Japanese War (1894–95), fought in the Russo-Japanese War (1904–05), and was deployed to Siberia during the intervention (1918–22).

The newly completed headquarters building on the Honmaru served the 4th Division from 1931 until 1940, when the Central District Army (中部軍) command moved in and the division headquarters relocated to a secondary site within the castle grounds near the Hōkoku Shrine area.

During the Second Sino-Japanese War, the 4th Division was stationed in Manchuria from February 1937 and soon engaged in operations across North China. In 1940–41 it participated in operations along the Han River, in the Yūnan campaign, and the First Battle of Changsha (September 1941).

With the outbreak of the Pacific War in December 1941, the 4th Division was committed to the Philippines campaign. It played a critical role in the Second Battle of Bataan (April 1942), breaking through American-Filipino defensive lines, and subsequently participated in the assault on Corregidor Island, contributing significantly to its capture. After returning to Japan for reconstitution, the division was redeployed in September 1943 to Sumatra for garrison duty, then transferred to Thailand under the 15th Army. The 4th Division was stationed at Lampang in northern Thailand when the war ended in August 1945.

Wartime and Postwar Transitions

During the Pacific War, the headquarters building on the Honmaru served the Central District Army command responsible for the defense of the Kinki and Chūbu regions. Remarkably, despite devastating American air raids on Osaka — particularly the firebombing raids of March and June 1945 that destroyed much of the city — the building escaped major damage, protected in part by its reinforced concrete construction and its location within the castle grounds.

After Japan's surrender in August 1945, the building was requisitioned by US occupation forces. From 1948 it was returned to Japanese use, first as the headquarters of the Osaka City Police (大阪市警視庁) and subsequently the Osaka Prefectural Police headquarters (大阪府警察本部).

Osaka City Museum Era (1960–2001)

In 1960, the building was converted into the Osaka City Museum (大阪市立博物館, Ōsaka Shiritsu Hakubutsukan), a museum dedicated to the history and culture of Osaka. For over four decades, the museum operated within the former military headquarters, its galleries filling the spaces that had once housed divisional staff offices. The museum closed permanently at the end of March 2001, when its collections and functions were transferred to the newly constructed Osaka Museum of History (大阪歴史博物館) located southeast of the castle.

MIRAIZA OSAKA-JO (2017–present)

After 16 years of vacancy, the building underwent extensive renovation while preserving its historical exterior. In October 2017, it reopened as MIRAIZA OSAKA-JO (ミライザ大阪城), a commercial complex featuring restaurants, cafés, souvenir shops, and entertainment facilities including the Kaiyodo Figure Museum in the basement. The name "MIRAIZA" combines the Japanese word "mirai" (未来, future) with the building's heritage, symbolizing the fusion of history and modernity. The rooftop terrace offers panoramic views of Osaka Castle's main tower.

Today, the building stands as a rare and remarkably well-preserved example of early Showa military architecture. Its Romanesque turrets and scratch-tile facade continue to evoke the era when the Imperial Japanese Army's presence dominated the Osaka Castle grounds — a tangible reminder of the militarization of Japanese society in the 1930s, now repurposed for leisure in a manner that few wartime military buildings have achieved.