To improve power supply efficiency, to improve reliability by miniaturizing a backup power supply, and to make a voltage rise faster at the time of starting a power supply.

A main power supply circuit PSM is of a switching system that obtains DC electric power with a pulse transformer T1, a semiconductor switch Q1 and a PWM control circuit CVC. The backup power supply PSS is powered by a super capacitor SC1 that is made to face the DC output of the main power supply circuit at reverse flow blocking diodes D4, D5. The super capacitor is of a small capacity type that supplies electric power during a period of time only required for the shutting down process of a load device. The super capacitor is charged by DC power obtained from AC power supplied from the secondary winding of the pulse transformer. The charging of the super capacitor includes the control of a charging current with a trailing overcurrent protective circuit.