PURPOSE: To allow an MOSFET to switch at a high speed by connecting the 1st photoelectromotive voltage generating element for driving and the 2nd photoelectromotive voltage generating element for reducing turn-on time in parallel between the gate and source of the MOSFET.

CONSTITUTION: The 1st photoelectromotive voltage generating element 24 for FET driving and the 2nd photoelectromotive voltage generating element 25 for reducing turn-off time are connected in parallel between the gate and source of an MOSFET21; and the 1st element 24 is operated by the 1st light source 26 and the 2nd element 25 is operated by the 2nd light source 27 on or just before the completion of said operation. Consequently, a reverse bias is applied between the gate and source of the FET21 to discharge a gate capacitance.