A transistor (48) is switched on or off to thereby cause a counter electromotive force on a coil (42) so that a high voltage is charged to a capacitor (46) via a zener diode (44). As a breakdown current of the zener diode (44) flows to the ground via the transistor (48), a voltage lower than a predetermined voltage is applied to the transistor (48). In addition, as the breakdown current of the zener diode (44) does not flow directly to the ground, variance of a boosted voltage can be prevented.