PURPOSE: To prevent spark discharge by inputting a current signal sent from a current detector to a thyristor controller and operating an electrostatic precipitator while predicting and preventing generation of spark discharge thereby.

CONSTITUTION: In a controller 20 having a thyristor 2 which is provided to the DC high-voltage power source devices 3-5 for supplying DC high voltage to an electrostatic precipitator 6, a low-voltage AC power source is controlled while keeping charged voltage and current value as the feedback signals. The electrostatic precipitator is operated so that the phase angle of ignition of the thyristor 2 is regulated to constant value at every constant time. In the meantime, while keeping the current peak value in an initial period as reference among the above-mentioned feedback signals, the ensuing current peak value is compared. When this comparison value has reached the prescribed preset value or more, the phase angle of the thyristor 2 is controlled so that spark discharge is not generated in the ensuing peak. As a result, spark discharge can be previously prevented. Further charged operation is safely enabled and also deterioration of performance of the electrostatic precipitator can be prevented.