PURPOSE: To prevent the occurrence of irregularities in images when a transfer is performed and when a latent image is formed by completing the energizing actions of the contact/uncontact means of the transfer assisting members of all image forming parts performing the transfer, just before the transfer is started, and energizing a transfer material supporting member in the direction of an image carrier.

CONSTITUTION: The contact/uncontact timing of backup members 60a - 60d is controlled. In other words, all backup members 60a - 60d are placed at contact/uncontact positions for pushing up the transfer material supporting member 8 before the first color toner image is transferred. After the last color toner image is completely transferred., all backup members 60a - 60d are placed at separate positions apart from the transfer material supporting member 8. Therefore, the contact/uncontact of the backup members 60a - 60d of the other image forming parts cannot be carried out with respect to the transfer material supporting member 8 when the transfer is performed on some image forming parts. Thus, the irregularities in the images when the transfer is performed and when the latent image is formed, are prevented.