PURPOSE: To automatically switch to a standby power supply without depending on another control when a selected power supply fails while it is in service by selecting one of a plurality of power supplies and supplying power with the minimum power loss, and performing control so that a standby power supply which is not selected can output a standby voltage.

CONSTITUTION: When a battery B1 or B2 fails due to voltage drop, etc., or is disconnected from a connection terminal, a voltage detection circuit 12 or 22 detects the absence of power and inform an MPU 16 of it and at the same time shuts off the power of its own operational amplifier U1 or U2 and a power supply selection circuit 11 or 12. When the battery B1 is in service and the battery B2 is in standby, the control of the power supply selection circuit 11 disabling the power supply selection circuit 21 is lost if the battery B1 loses power, thus controlling the operational amplifier U2 and enabling an FET 2 to conduct electricity completely at the same time when the power supply selection circuit 21 instantly turns on the FET 4.