An isolated synchronous rectification type DC/DC converter, includes: a transformer including primary and secondary windings; a switching transistor connected to the primary winding; a synchronous rectifying transistor installed between the secondary winding and a ground line on the secondary side; a photocoupler including a light emitting device and a light receiving device; a feedback circuit driving the light emitting device such that an output voltage of the DC/DC converter approaches a target voltage; a primary side controller connected to the light receiving device and switching the switching transistor depending on feedback signal from the photocoupler; a synchronous rectification controller controlling the rectifying transistor; and a protection circuit including a temperature detection element, one end of the detection element connected to a drain of the rectifying transistor, the protection circuit configured to detect an overheated state of the rectifying transistor depending on a first signal generated by the temperature detection element.