To simplify a circuit and reduce a cost, by suppressing an arm short- circuit so that high speed switching cap be operated.

This two-crystal half bridge type regulator is constituted in a manner wherein both ends of two capacitors 3, 4 connected in series to both ends of two series connection semiconductor switches 1, 2 are connected in parallel, also a connection point of the two semiconductor switches 1, 2 and a connection point of the two capacitors 3, 4 are respectively connected to one end of a primary side coil of an output transformer 15. A beads-shaped core 14 is inserted in series to the semiconductor switch 1 in an arm part connecting the semiconductor switches 1, 2. A gate drive circuit of the semiconductor switches 1, 2, inputting a pulse from a pulse transformer 7 provided with two secondary side coils of different polarity, is alternately on/off controlled.