An insulated DC/DC converter includes: a transformer; a switching transistor; a rectifier circuit; a photocoupler; a feedback circuit configured to drive a light emitting element of the photocoupler such that an output voltage of the DC/DC converter approaches a target voltage; a primary side controller having a feedback terminal which is connected to a light receiving element of the photocoupler and receives a feedback signal from the photocoupler, a zero current detection terminal which receives a zero current detection signal corresponding to a voltage generated at one end of an auxiliary winding of the transformer, and a pulse modulator of a quasi-resonant mode configured to generate a pulse signal depending on the feedback signal and the zero current detection signal; and a starting control circuit which, in start-up of the DC/DC converter, electrically affects the zero current detection terminal such that an OFF time of the switching transistor lengthens.