PURPOSE: To prevent a harmful residual charge from occurring by increasing the capacity of a first capacitor which is connected in parallel with a charge power supply out of first and second capacitors which are connected in series to be larger than that of the second capacitor.

CONSTITUTION: When a semiconductor switching element 7 conducts current and then the charge of a first capacitor 4 is inverted after the first capacitor 4 and a second capacitor 5 are charged, they are charged initially so that the amount of charge of the first capacitor 4 is equal to that of the second capacitor 5. Namely, capacities C1 and C2 are set so that the capacity of the first capacitor 4 and that of the second capacitor 5 satisfy an equation (CIE2-R(t).I (t)2)/2=(C2E2)/2 (E; power supply voltage, C1>C2) when a synthesis resistor of the semiconductor switching element 7, a first reactor 3, and the first capacitor 4 is set to R(t) and an inversion current is set to i(t).