A control circuit for controlling a subsidiary (high-side) switching device in an insulating electric power converter having a half-bridge structure includes a first series circuit of a resistor and a diode, a second series circuit of a resistor and a diode, and a transistor. The control circuit turns the subsidiary switching device ON and OFF using the voltage generated in a transformer winding for a signal voltage. The control circuit also turns the transistor ON and OFF with the voltages generated in the first and second series circuits so as not to make the gate voltage of subsidiary switching device exceed the gate breakdown voltage thereof in all the operation modes.