A motor starts its rotation, and a clutch is operated to drive both of a first driving force transmission system for color printing, including drum gears, and a second driving force transmission system for monochrome printing, including a drum gear. The clutch is disengaged to stop the first system when a first sensor detects a mark on one photosensitive drum of the first system at the same time that a second sensor detects a mark on one drum driving gear of the first system. Next, when a third sensor detects a mark on another photosensitive drum of the second system at the same time that a fourth sensor detects a mark on another drum driving gear of the second system, the clutch is reengaged. Consequently, positionings of all the photosensitive body drums (drum gears) for color printing to the photosensitive drum (drum gear) for monochrome printing are performed without causing positional differences by inertia at the time of stop.