A station-building power supply device that supplies power to a station load using both AC power supplied from an AC system and surplus regenerative power from trains, the station-building power supply device comprising: a feeding-voltage detection unit that detects a feeding voltage; a first power conversion unit that performs DC/DC power conversion in both directions between a feeder and a capacitor unit that stores therein the surplus regenerative power; a second power conversion unit that converts DC power supplied from the capacitor unit to AC power and supplies the AC power to the station load; a state of charge detection unit that detects a state of charge, abbreviated SOC hereafter, of the capacitor unit; and a control unit that controls the first power conversion unit and the second power conversion unit on the basis of the feeding voltage and the SOC, wherein the control unit controls, when the feeding voltage exceeds a predetermined first voltage threshold, the first power conversion unit such that power is supplied from the feeder to the capacitor unit, controls, when the SOC exceeds a predetermined first SOC threshold, the second power conversion unit such that power is supplied from the capacitor unit to the station load, and controls, when the SOC falls below the predetermined first SOC threshold, the second power conversion unit such that power supplied from the capacitor unit to the station load is gradually decreased to be substantially zero. a feeding-voltage detection unit that detects a feeding voltage; a first power conversion unit that performs DC/DC power conversion in both directions between a feeder and a capacitor unit that stores therein the surplus regenerative power; a second power conversion unit that converts DC power supplied from the capacitor unit to AC power and supplies the AC power to the station load; a state of charge detection unit that detects a state of charge, abbreviated SOC hereafter, of the capacitor unit; and a control unit that controls the first power conversion unit and the second power conversion unit on the basis of the feeding voltage and the SOC, wherein the control unit controls, when the feeding voltage exceeds a predetermined first voltage threshold, the first power conversion unit such that power is supplied from the feeder to the capacitor unit, controls, when the SOC exceeds a predetermined first SOC threshold, the second power conversion unit such that power is supplied from the capacitor unit to the station load, and controls, when the SOC falls below the predetermined first SOC threshold, the second power conversion unit such that power supplied from the capacitor unit to the station load is gradually decreased to be substantially zero. controls, when the feeding voltage exceeds a predetermined first voltage threshold, the first power conversion unit such that power is supplied from the feeder to the capacitor unit, controls, when the SOC exceeds a predetermined first SOC threshold, the second power conversion unit such that power is supplied from the capacitor unit to the station load, and controls, when the SOC falls below the predetermined first SOC threshold, the second power conversion unit such that power supplied from the capacitor unit to the station load is gradually decreased to be substantially zero.