PURPOSE: To make it possible to prevent the deterioration of the polarization of a piezoelectric element by changing the cross-sectional area of an electromagnetic circuit in response to the positive or negative maximum voltage applied on the piezoelectric element, changing the saturation amount of an inductor, and controlling the undershoot to the negative side.

CONSTITUTION: A charging coil 14' and a discharging coil 17' have the magnetic circuit different from the conventional circuit. Namely, a magnetic core 2 has the first magnetic circuit part (MD) 3 having a cross-sectional area S1 and a magnetic path length l1 and the second MD 4 having a cross-sectional area S2 (S2<S1) and a magnetic path length l2 in series. The second MD 4 is constituted so that the area S2 is changed and the second MD 4 is saturated before the first MD 3. In details, a feedback control circuit 18 measures the maximum value Vp+ of the positive voltage and the maximum value Vp- of an applied voltage Vp of a piezoelectric element 101. The indicated values of them are compared, and the small cross-sectional area S2 of the magnetic circuit of the charging coil 14' and the discharging coil 17' undergoes feedback control based on the deviation.