PURPOSE: To always collect dust stably at high dust collecting efficiency, by detecting a load state based on the physical properties of dust in an electric dust collector on the basis of an apparent resistance value between electrodes and permitting the electrodes to perform optimum charging corresponding to the load state.

CONSTITUTION: The voltage between the electrodes 10a, 10b of an electric dust collector 10 is detected by a voltage detector 1 through voltage dividing resistors 11, 12 and a load current is detected by a current detector 2 through CT13 inserted into an power source circuit. Both detected values are inputted to an operation means 3 to operate the apparent resistance value between electrodes 10a, 10b. The operated value is inputted to a comparator 4 and compared with a preset reference value and, when it is larger than the reference value, an intermittent charging control means 5A is selected while a continuous charging control means 5B is selected when it is smaller than said reference value to determine a charging control mode. By this operation, a phase shifter 5 is operated to control the thyristor phase of a voltage controlling thyristor apparatus 6 and charging adapted to the load of the electric dust collector 10 is automatically performed to operate said dust collector with stable high dust collecting efficiency.