The purpose of the invention is to obtain a station building power supply device capable of satisfying both the effective use of excess regenerative power and the supply of emergency power to station buildings, with a configuration facilitating both installation and handling in accordance with the amount of power necessary for each station building and matching the amount of power necessary for each station building and also to obtain a method for controlling the station building power supply device. The station building power supply device (1) installed in each of the station buildings (2) and supplying a low-voltage AC power to the station building facilities (3) of each of the station buildings (2) has a regenerative mode and a backup power supply mode. In the regenerative mode, when regenerative power regenerated from a train (7) to an overhead line (8) becomes surplus, the surplus regenerative power supplied through the overhead line (8) and the power supplied from a high-voltage distribution line (5) are combined and the low-voltage AC power corresponding to the ordinary power consumption amounts of the station buildings (2) is supplied. In the backup power supply mode, when the high-voltage distribution line (5) breaks down, the low-voltage AC power corresponding to the emergency power amounts of the station buildings (2) is supplied using the power supplied from the overhead line (8).