In a color laser printer, in which phases of periodic variations in abutting position speeds of photoconductor drums and timings of transfer of toner images to a medium are synchronized, the variation in speed of the medium due to periodic variations of the abutting position speeds are reduced. In the color laser printer, in which a medium is moved while being abutted by four photoconductor drums, and thereby toner imaged carried by the photoconductor drums are superimposingly transferred to the medium, the photoconductor drums are arranged such that the distance between neighboring two abutting positions of the photoconductor drums with the medium is three or five times a value obtained by dividing a circumferential length of the outer circumferential surface of each of the image carriers by the number of the carriers.