The power supply apparatus includes a transformer having a primary and secondary sides, a first line and a second line to which an AC voltage is input from an AC power supply, a rectifying and smoothing unit that rectifies and smoothes the AC voltage, a shut-off unit provided between the first line and the rectifying and smoothing unit, a switching element that switches a current from the rectifying and smoothing unit to the primary side of the transformer, a first overvoltage detection unit that outputs a voltage corresponding to the AC voltage, a second overvoltage detection unit that outputs an overvoltage detection signal; and a control unit that controls the shut-off unit to shut off the input of the AC voltage.