PURPOSE: To adjust easily and freely the width of a rubber roller with which the rubber roller is pressed to a photoreceptor, by providing a cavity in the conductive rubber roller, which adjusts the initial potential of the use of the photoreceptor to change the outside diameter of the rubber roller by a compressed fluid.

CONSTITUTION: A cavity 3 is provided in a conductive rubber roller which adjusts potential unevenness of a photoreceptor in the initial stage of the use by application of a high voltage and is pressed to the photoreceptor. When a high-pressure gas or liquid is flowed into this cavity 3 through a roller shaft 1, the outside diameter of the rubber roller is changed; and thus, the width of the rubber roller with which the rubber roller is pressed to the photoreceptor is adjusted easily and freely, and the potential of the photoreceptor is set well in the initial stage.