In the case that the electrical resistance of the contact transfer body 15 varies with the variation of the operating environment, the variation of the value of the electrical resistance is detected by the transfer body resistance detecting section and the applying time of the transfer cleaning bias is appropriately varied by the cleaning bias control section 23 based on the detected result. When the operating environment varies to a high temperature condition or to a high humidity condition, the applying time of the transfer cleaning bias is shortened by the cleaning bias control section 23, and the continuation of the conventional overdischarged condition is avoided. Therefore, even when the value of the electrical resistance of the contact transfer body 15 varies according to the variation of the operating environment, the image can be formed with a high degree of quality and the shortening of the service life of the image carrier can be prevented.