It was obvious that the Japanese Hunter's Association HQ used to be located in the central district of Tokyo, commonly seen as the beating heart of Japan.
But, that was only until the Giants began pouring out from the Gate, of course.
On that fateful day, as Shinjuku's rank S Gate morphed into a full-on dungeon break, the President of the Japanese Association Matsumoto Shigeo managed to save his life through the sacrifices of the Hunters and soldiers stationed on the frontlines. He hurriedly re-established the HQ of the Association in the city of Osaka's local government offices.
There was only one reason why he chose this city, rather than some others located on the north-west of the country.
He figured that, if they failed to protect the city of Osaka and its 2.66 million inhabitants, then there was no hope for Japan, either. In other words, this was their last defence line – their Maginot Line.