An image forming apparatus includes optical sensor units provided opposed to an image bearing member; light blockers provided between the image bearing member and the optical sensor units, respectively, the blockers being movable between locking and exposing positions, respectively; calibration members provided where the light is incident when the blockers are in the blocking positions, respectively, the calibration members calibrating data acquired by the optical sensor units; a movable link connected with the blockers; a switching portion for switching the blockers between the exposing and blocking positions; and a positioning portion for determining the blocking position, the positioning portion being disposed at a position where at least one of the blockers is contacted to the positioning portion when the blockers are switched from the exposing positions to the blocking positions.