According to an embodiment of the present invention, color drift is prevented from occurring in a color image by detecting a position of a rotation informing mechanism integrally held by one of first, second, and third gears to detect a phase change in full rotation of at least one gear, detecting a position of a rotation informing mechanism integrally held by a fourth gear to detect a phase in full rotation, and setting, on the basis of the detected one phase change and the detected other phase change, phases during start of rotation of the respective gears such that a phase of a fourth image held by an image holding member rotated by the fourth gear coincides with phases of first to third images held by respective image holding members rotated by the first, second, and third gears.