A power switch circuit includes a first switch transistor connected to a main power supply, which supplies a first voltage, a second switch transistor connected in series to the first switch transistor and to a backup power supply, which supplies a second voltage. A switch control unit controls activation and deactivation of the first and second switch transistors so that either one of a voltage corresponding to the first voltage and a voltage corresponding to the second voltage is selectively output to a connection node between the first and second transistors. The first switch transistor includes a first diode, which is formed so that a direction from the main power supply toward the connection node defines a forward direction, and a second diode, which is formed so that a direction from the connection node toward the backup power supply defines a forward direction.