Rectifying elements are connected in parallel with switching elements 4-1 through 4-4 in a switching section 4 for the lower voltage side and rectifying elements are connected in parallel with switching elements 5-1 through 5-4 in a switching section 5 for the higher voltage side. An LC resonant circuit 6 is provided between the winding wire 3-2 for the high-voltage side in the transformer 3 and the switching section 5 for the higher voltage side; currents which flows on the primary side and the secondary side is changed into a sinusoidal one; and switching is executed in the vicinity of the zero crossing points of the currents.