In an image forming apparatus, toner images of respective colors are sequentially formed on an image carrier. A primary image transfer unit sequentially transfers the toner images from the image carrier to an intermediate image transfer belt one above the other, thereby producing a composite color image on the belt. A secondary image transfer unit transfers the composite color image from the belt to a sheet or similar transfer material. When the belt makes a turn without image transfer, i.e., idles, an electric field output lower than an electric field output preselected for image formation is applied to the primary image transfer unit.