PURPOSE: To enable exchanging of a sheet at low cost and to prevent occurrence of insulation deterioration by using the sheet converted to electret for a dust collecting electrode.

CONSTITUTION: Dusts contained in dust-laden air which is flowed into the inside of a casing 11 are charged by a corona discharge between a discharging wire 12 impressed with high voltage and counter electrodes 13. In the dust collecting part wherein the sheets 14 converted to electret are faced at intervals, the charged dusts are collected to sheets 14 by the coulomb force operating between charged dusts and electric charge of sheets 14. In this way, since the dust collecting part is formed by facing the sheets 14 converted to electric along an air current in the direction of rectification at intervals, the apparatus composed of an inexpensive and simple structure is obtained.