While description is given above of a case where the transfer belt 7, in step S4, are moved apart from the photosensitive drums 3 b to 3 d except the photosensitive drum 3 a, a similar effect can be obtained by turning off a transfer bias to be applied to the transfer rollers 6 b to 6 d or by setting a transfer bias to a proper value without moving the transfer belt 7 apart from the photosensitive