PURPOSE: To provide a concrete constitution for reducing the deterioration of a photoreceptor drum caused by the soiling of a corona discharge wire by an electrostatic dust collecting action due to the unnecessary exposure of the corona discharge wire because of the size of a paper sheet and the direct soaring up/sticking of a corona ion from the corona discharge wire to the photoreceptor drum.

CONSTITUTION: This corona discharger is constituted in such a manner that cover members 4a and 4b moved in opposite directions to each other by the rotation of a motor 5 are arranged between a photoreceptor member 1 and the corona discharge wire and the motor 5 is rotated in accordance with the size of a transfer member (paper sheet) 10 to be used, to move the cover members 4a and 4b via a gear 6 and a rack 7 and change the size of an opening part for discharge formed between both cover members 4a and 4b, so that the corona discharge wire is never unnecessarily exposed.