A MOSFET at an input side controls the operation of a current mirror circuit in accordance with a level change of a PWM signal applied to its gate. When the current mirror circuit operates, a current generated by a current source flows as a mirror current so that a current flows to discharge electricity charged in a capacitance between a gate and a source through a gate of a MOSFET at an output side. When the current mirror circuit stops its operation, a current flowing from the current mirror circuit through the current source is supplied to the gate of the MOSFET at the output side.