A mark detector optically detecting a scale having multiple marks formed successively at predetermined intervals along the moving direction of an endless belt member, and outputting an electrical signal corresponding to the presence or absence of the marks when the endless belt member moves is disclosed. The mark detector includes a light illumination part configured to illuminate the light illumination surface of the endless belt member on which surface the scale is formed with parallel light rays; a light receiving part configured to receive reflected light from the light illumination surface; and a variation prevention part configured to prevent a variation of the light illumination surface. The variation prevention part includes a holding member configured to hold the endless belt member in the vicinity of the light illumination surface movably in the moving direction from the exterior surface side and the interior surface side of the endless belt member.