A control unit controls an on/off operation of a switching unit such that a first DC pulse voltage is applied across an electrode gap from a pulse-voltage generation unit adjusted to a first impedance, and when a detection unit detects a preliminary discharge, a second DC pulse voltage is applied across the electrode gap from the pulse-voltage generation unit adjusted to a second impedance, which is lower than the first impedance, so as to generate a main discharge in the electrode gap, and during an idle time since the main discharge is completed until the next preliminary discharge is started, a third DC pulse voltage that is equal to or higher than a discharge arc voltage is applied across the electrode gap from the pulse-voltage generation unit adjusted to a third impedance, which is higher than the first impedance.