PURPOSE: To separate surely a transfer material from an image carrier, by providing an insulating member along the aperture end part in the longitudinal direction of feeding the transfer material side of a separating charger and varying the aperture width of the separating charger by this insulating member.

CONSTITUTION: An insulating member 8 is provided along the aperture end part in the longitudinal direction of a separating charger 3 to control the electric field for discharging of a transfer material 2. The insulating member 8 changes the width in the advance direction of the transfer material or is moved in this direction to vary the aperture width of the separating charger 3, thereby adjusting the value of the field intensity of AC corona and correcting the inclination of the separating charger 3 in the longitudinal direction.