To reduce the crossectional area of a magnetic core in a pulse transformer by detecting the magnetic density of the transformer due to the excitation of ripple voltage between a dust collection electrode and an electric discharge electrode and applying a control signal to a power control switch on a high voltage pulse generation circuit for making the switch when the magnetic flux density is reduced from a maximum value by a specified value.

A power control switch S2 on a high voltage pulse generation circuit 5 is turned ON by a control signal from a control part 5B, and in this case, the magnetic flux density of a magnetic core in a pulse transformer PT is detected, in terms of voltage, by voltage detection resistor R1 and R2. Further, the detected voltage is integrated by an integrating circuit IN to obtain a voltage time product corresponding to the actual magnetic flux density. Then, in a comparator CP, a voltage signal from a subtraction circuit SS is compared to a voltage corresponding to the magnetic flux density of the magnetic core in the pulse transformer PT. When the latter is lower than the former, an output signal is sent to a control signal generator CC. The control generator CC receives the output signal and supplies a control signal to the power control switch S2 so that the switch S2 is turned ON.