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 generated by trains, the station-building power supply device comprising: a feeding-voltage detection unit that detects a feeding voltage; an SOC, State Of Charge, detection unit that detects an SOC of a capacitor unit that stores therein the surplus regenerative power; a first power conversion unit that performs DC/DC power conversion in both directions between a feeder and the capacitor unit; 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; 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 the first power conversion unit such that power is supplied from the feeder to the capacitor unit when the feeding voltage exceeds a predetermined first voltage threshold, controls the second power conversion unit such that power is supplied from the capacitor unit to the station load when the SOC exceeds a predetermined first SOC threshold, and controls the first power conversion unit such that power is supplied from the capacitor unit to the feeder when the feeding voltage falls below a second voltage threshold that is smaller than the first voltage threshold and the SOC exceeds a second SOC threshold that is larger than the first SOC threshold. a feeding-voltage detection unit that detects a feeding voltage; an SOC, State Of Charge, detection unit that detects an SOC of a capacitor unit that stores therein the surplus regenerative power; a first power conversion unit that performs DC/DC power conversion in both directions between a feeder and the capacitor unit; 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; 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 the first power conversion unit such that power is supplied from the feeder to the capacitor unit when the feeding voltage exceeds a predetermined first voltage threshold, controls the second power conversion unit such that power is supplied from the capacitor unit to the station load when the SOC exceeds a predetermined first SOC threshold, and controls the first power conversion unit such that power is supplied from the capacitor unit to the feeder when the feeding voltage falls below a second voltage threshold that is smaller than the first voltage threshold and the SOC exceeds a second SOC threshold that is larger than the first SOC threshold. controls the first power conversion unit such that power is supplied from the feeder to the capacitor unit when the feeding voltage exceeds a predetermined first voltage threshold, controls the second power conversion unit such that power is supplied from the capacitor unit to the station load when the SOC exceeds a predetermined first SOC threshold, and controls the first power conversion unit such that power is supplied from the capacitor unit to the feeder when the feeding voltage falls below a second voltage threshold that is smaller than the first voltage threshold and the SOC exceeds a second SOC threshold that is larger than the first SOC threshold.