PURPOSE: To surely blow off deposited dust without decreasing the operation efficiency by estimating the leak current flowing on the surface of an insulator on the basis of the detected result of a leak current flowing on the surface of an insulating material in the same environment as the insulator.

CONSTITUTION: With respect to the method for removing dust deposited on an insulator for supporting an electrode in an electric dust collector, an insulating material 201 for detecting a leak current which is similar to the insulator 101 for supporting a collecting unit and a detector 202 for detecting a leak current flowing on the surface of the insulating 201 are connected in series and provided in the vicinity of the insulator 101. When that the leak current reaches a criterion is discriminated by a control part 205, the blowing off of the dust collected on a charged unit and the collecting unit are automatically carried out.