An image forming apparatus includes a movable member that moves cyclically in synchronization with an image forming process and a position detector. The position detector includes a scale, an image sensor, a signal processor, and a position computing unit. The scale is attached to the movable member and includes a plurality of optical marks formed in line at a substantially constant interval. The image sensor captures data relating to at least one optical mark at a time. The signal processor computes a coordinate of the optical mark with respect to a reference position. The position computing unit computes a distance traveled by the optical mark based on a number of the optical marks that pass the reference position and the coordinate of the optical mark with respect to the reference position.