To ensure high reliability without causing any instantaneous power supply interruption by reducing the probability that respective systems break down simultaneously in electric power units in which power supply units are multiplexed.

Each of a plurality of power supply units includes a voltage converter and a diode inserted between the voltage converter and a power supply line with power supply set in a forward direction. Voltage of the power supply unit to be supplied to equipment is variable. The electric power supply unit includes a control part for controlling a supply voltage of the plurality of power supply units. The control part performs control so that the supply voltage becomes higher than that of the power supply unit in the other standby systems with one power supply unit of the plurality of power supply units as a current system, and changes overs the power supply unit of the current system and that of the standby system at a preset timing.