Station building auxiliary power supply device

The present invention is a station building auxiliary power supply device that generates electric power to be supplied to a load in a station building by converting electric power that is supplied from an overhead wire when the voltage of the overhead wire is higher than a predetermined threshold value, and is equipped with: a first average calculation section (31) that calculates an average value of the overhead wire voltage at fixed time intervals; a second average calculation section (32) that calculates an average value of overhead wire voltages that are higher than the first average value, that is, the aforementioned average value, among multiple overhead wire voltages that are detected during the fixed period of time; a third average calculation section (33) that calculates an average value of the overhead wire voltage per day; and a comparator (34) that selects either one of the second average value, that is, the average value calculated by the second average calculation section, and the third average