PURPOSE: To perform the miniaturization and the reduction in price by connecting successively a unit constituted of a solid-state switching element, a gate circuit and a DC/DC converter in cascade.

CONSTITUTION: A unit 5 is constituted of an insulation type DC/DC converter 1, a gate circuit 2, a solid-state switching element 3 and an optically coupled isolator 4. Plural units 5 are connected successively an input output element in cascade. Thus, from a direct current power 6 and a gate pulse power 7 provided on a low voltage side, the power of each gate circuit and a pulse signal for switching can be successively communicated with respective units 5 up to the upper-most step unit at the high voltage side and the high voltage switching action can be performed. By such constitution, the number of units may be determined in accordance with the service voltage. the direct current power is miniaturized and the reduction in the price can be attained.