The feedback IC is provided at the secondary side of the DC/DC converter and is coupled to the photo coupler. The error amplifier amplifies an error between a voltage detection signal according to an output voltage of the DC/DC converter and a target voltage, and draws a current according to the error from the input side of the feedback photo coupler via the photo coupler connection terminal. The abnormal detection circuit asserts an abnormal detection signal when an abnormal condition in a secondary side of the DC/DC converter is detected. The protection circuit is coupled to the photo coupler connection terminal, and acts on the feedback photo coupler via the photo coupler connection terminal so that a feedback operation by the error amplifier is invalid and an on-period of the switching transistor is shorten.