An image forming apparatus: includes a developing member to be movable between a development position where the developing member contacts an image bearing member and a separated position where the developing member is separated from the image bearing member; and a control portion controls the number of rotations of a drive portion by causing a light source to emit a light beam when a rotating polygon mirror starts to rotate, and detecting the light beam emitted from the light source by a detecting portion, wherein when the rotating polygon mirror starts to rotate, the light beam does not enter an image region of the image bearing member and the light beam is detected outside the image region of the image bearing member by the detecting portion, in a state of the developing member having moved from the separating position to the development position.