PURPOSE: To transfer and separate stably transfer forms different in thickness and prevent the degradation of a photoreceptor, by changing the irradiation light from a pre-transfer exposure lamp to the photoreceptor on a basis of information of a means which detects the thickness of a transfer form.

CONSTITUTION: When the control signal of a controlling circuit 15 corresponds to a transfer paper thickness of 50g/m2, a changeover switch S is connected to a resistance Rb, and a pre-transfer exposure 10 performs the pre-transfer exposure with 30lux.sec; and when said control signal corresponds to a transfer form thickness of ≥65g/m2, the switch S is connected to a resistance Ra, and the pre- transfer exposure lamp 10 performs the transfer exposure with 15lux.sec. Thus, transfer forms different in thickness are always transferred and separated stably even under a constant condition where the discharge current of a transfer electrode is 30μA. In this case, the transfer form thickness may be detected by the change of the electrostatic capacity between both poles of a capacitor.