PURPOSE: To form color images of high quality while preventing a color shift by interrupting a transfer process and performing a latent image developing process again when the detected moving speed of a transfer material support body which is driven independently of an image carrier goes beyond a permissible range.

CONSTITUTION: A photosensitive drum 1 as the image carrier and a transfer drum 4 as the transfer material support body are rotated by driving motors as mutually independent driving sources and a color toner image formed on the drum 1 by a developing device is transferred successively to a transfer material through the transfer charger 9, etc., of the drum 4 to obtain a color image. The moving speed of this drum 4 is detected on the basis of the passing detection of a magnet 33 at the front end position of the drum 4 by a front end detecting element 32 and a detecting element 34 and when the moving speed is not within the permissible range, the operation of the charger 9 is stopped. Then, image transfer of the color is not performed and a latent image formation and development processing is performed again on the drum 1. Thus, a color shift in an image is prevented and the color image of high quality is formed.