A cam transmits and blocks light emitted from a light emitting portion. The cam moves to: a first position at which first and second cartridges are attachable to and detachable from a housing; a second position at which a first development roller contacts a first photosensitive drum, the second position being a position at which the cam has moved in a first direction from the first position; and a third position at which the first development roller is separated from the first photosensitive drum and a second development roller is separated from a second photosensitive drum, the third position being a position at which the cam has moved in the first direction from the second position. During an initial operation, based on a detection result of the sensor, a controller determines which of the first position, the second position, and the third position the cam is located at.