Provided is a backup power supply that efficiently supplies power under the control of a microcomputer. The backup power supply is charged by a rectifier for generating power of a predetermined voltage, and supplies power to a load device operating with power supplied from the rectifier when the supplied power is insufficient. The backup power supply includes a secondary battery to be charged with power from the rectifier, a control unit for controlling charging and discharging of the secondary battery, and an input/output voltage detection unit for detecting the output voltage from the rectifier. The control unit stops charging of the secondary battery or reduces a charging current flow of the secondary battery when the output voltage drops to a first threshold voltage or less.