In the FIG. 6 embodiment, control circuit 1 is connected as depicted and adapted to receive a signal corresponding to the total current of the primary side through a current transformer CTl and the overcurrent detection circuit CS1, and also a signal corresponding to the load current in secondary coil n23 through a current transformer CT4 and overcurrent detecting circuit CS2. Switching elements Q1, Q; are turned OFF, for exam ple, in response to overcurrent detecting signal from either one of these overcurrent detecting circuit CS1,CS2. It is also possible to use a common overcurrent detecting circuit.