PURPOSE: To control concentration of sooty dust and save energy, by adjusting charging rate of an electrostatic precipitator when detected value of concentration of sooty dust in outlet discharged gas out of set range.

CONSTITUTION: Concentration of sooty dust of outlet discharged gas sampled by a sampling tube 8 is measured in a sooty dust concentration detecting section 7 and converted to a signal 9 and inputted to an energy saving controller 5. The value is compared with preset range of concentration and the result of calculation is sent to a power pack controlling board 3 as a parameter setting signal 10 of charging rate, and charging rate in the power pack 2 is controlled by intermittent charging. By this way, concentration of sooty dust in outlet discharged gas is maintained within a specified range, and energy saving is attained.