To reduce power consumption during standby waiting to as closely to zero as possible.

A switch 11 and a backup capacitor C11 are provided respectively between an AC power supply 1 and a transformer 12 for connecting or disconnecting the AC power feeding path to the transformer 12 from the AC power supply depending on the on or off condition, and moreover a supply means 131 for supplying the operating power to a microprocessor unit(MPU) 16 is also provided, and the MPU 16 is also provided to decide whether or not the backup capacitor C11 should be charged when the switch 11 is turned off, then is turned on the switch 11, when it is decided that the backup capacitor C11 should be charged, and thereafter to decide whether or not the backup capacitor C11 has completed the charging and then to turn off the switch 11 when it is decided that the backup capacitor C11 has completed the charging.