Imperial Palace / Obunko Annex

The Obunko Annex (御文庫附属室, Obunko Fuzokushitsu), formally known as the Obunko Fuzoku-ko (御文庫附属庫), is an underground air-raid shelter located within the Fukiage Gardens (吹上御苑) of the Imperial Palace in Chiyoda Ward, Tokyo. It stands as the single most important site of Japan's decision to surrender in World War II — the place where Emperor Showa (Hirohito) issued his "Sacred Decision" (聖断) to accept the Potsdam Declaration and end the conflict.

CONSTRUCTION AND STRUCTURE

The bunker was constructed by the Imperial Japanese Army's Fortification Department headquarters (陸軍築城部本部), with work commencing on August 12, 1941, and the completed facility transferred to the Imperial Household Ministry (宮内省) on September 30, 1941. Located approximately 100 meters northeast of the Obunko (御文庫) — the Emperor and Empress's main wartime residence — the two structures were connected by an underground passage added in 1945 (since filled in).

The facility has a total floor area of 631.5 square meters and is buried approximately 10 meters underground. Its reinforced concrete exterior walls are 3 meters thick, with an additional protective soil and concrete layer bringing the total shielding to 14.3 meters — making it one of the most heavily fortified air defense installations in wartime Japan. The bunker features a U-shaped layout with two entrance corridors (east and west), each 2 meters wide, designed to deflect blast waves from nearby bomb impacts. Eight iron doors, ranging from 25 to 35 centimeters thick, with double wooden-and-iron construction, seal the interior chambers.

The interior comprises five main rooms: the conference hall (会議室, 60 m², 6m × 10m — the largest space), the Emperor's rest quarters (御休所, 24 m²), an adjacent chamber (次室, 24 m²), a mechanical room (機械室, 78 m²), and a communications room (通信室, 18 m²). The rooms are connected by corridors with anterooms and three lavatories, including one designated exclusively for the Emperor (御厠). The mechanical room housed American-made generators supplemented by four bicycle-powered backup generators, along with temperature and humidity control systems and communications equipment linking to external military facilities.

THE SACRED DECISION — AUGUST 1945

By the summer of 1945, Japan's military situation was desperate. Following the atomic bombings of Hiroshima (August 6) and Nagasaki (August 9), and the Soviet declaration of war (August 8), Prime Minister Kantaro Suzuki convened an emergency session of the Supreme War Direction Council (最高戦争指導会議).

On the night of August 9–10, 1945, the Supreme War Direction Council met in the Obunko Annex conference room but reached a deadlock: three members (Prime Minister Suzuki, Foreign Minister Shigenori Togo, and Navy Minister Mitsumasa Yonai) favored accepting the Potsdam Declaration with the sole condition of preserving the Emperor's sovereignty (kokutai, 国体), while three others (Army Minister Korechika Anami, Army Chief of Staff Yoshijiro Umezu, and Navy Chief of Staff Soemu Toyoda) demanded additional conditions including no occupation, self-disarmament, and Japanese-conducted war crimes trials.

Unable to break the impasse, Suzuki took the extraordinary step of requesting the Emperor's personal intervention — the "Sacred Decision" (聖断). In the early morning hours of August 10, Emperor Hirohito spoke in the cramped underground conference room, declaring his agreement with the Foreign Minister's position to accept the Potsdam Declaration with only the condition of preserving the kokutai. This was the first Seidan.

However, when the Allied reply (the Byrnes Note) arrived on August 12 stating that the Emperor's authority would be "subject to" the Supreme Commander of the Allied Powers, hardliners in the military again resisted. The Cabinet deadlocked once more.

On the morning of August 14, a second and final Imperial Conference (御前会議) convened in the same underground conference room. Emperor Hirohito, visibly moved, reaffirmed his decision to surrender. Speaking to the assembled ministers and military leaders, he expressed his willingness to make a personal radio broadcast to the nation. This second Seidan was definitive. By midday, the fate of the Japanese Empire was sealed in this 60-square-meter room beneath the Imperial Palace gardens.

That same evening, the Emperor recorded the Gyokuon-hoso (玉音放送, "Jewel Voice Broadcast") in the Imperial Household Ministry building above ground. During the night of August 14–15, a group of junior Army officers attempted a coup d'état (the Kyujo Incident, 宮城事件) to seize the recordings and prevent the broadcast, but failed. The surrender announcement was broadcast to the nation at noon on August 15, 1945.

POSTWAR AND PRESENT DAY

After the war, the Obunko Annex fell into disuse and was sealed. Over the decades, the underground structure deteriorated significantly. When the Imperial Household Agency released photographs and video footage on August 1, 2015 — the 70th anniversary of the war's end — inspectors found extensive concrete spalling, cracks, water infiltration, soil ingress, and evidence of wildlife (including masked palm civets). Interior temperatures measured 16–17°C with humidity exceeding 80%.

The Obunko Annex lies deep within the Fukiage Gardens, an area closed to the general public. The Imperial Palace grounds that visitors can access include the Imperial Palace Outer Gardens (皇居外苑, Kokyo Gaien), the East Gardens (皇居東御苑), and the general visitation route. The Outer Gardens plaza offers the iconic view of the Nijubashi bridges and the Fushimi Yagura turret — the most recognizable image of the Imperial Palace. General visitations (一般参賀) on New Year's Day and the Emperor's birthday allow the public to cross the Nijubashi and enter the inner grounds, though the Fukiage area remains off-limits.

For visitors seeking to understand the significance of this site, the recommended approach point is the Nijubashi Bridge area in the Outer Gardens (二重橋前), approximately 1.5 km southeast of the actual Obunko Annex location within the Fukiage Gardens. The Outer Gardens are freely accessible and offer the closest public vantage point toward the inner palace grounds where the surrender decision was made.