At least one exemplary embodiment is directed to an image forming apparatus, which includes: a first to fourth image bearing members on which toner images of four colors of yellow, magenta, cyan, and black are formed respectively; a first rotatable polygon mirror for deflecting and scanning laser beams on the first to fourth image bearing members so as to form electrostatic latent images on the first to fourth image bearing members; a detachable fifth image bearing member on which a toner image of a color different from the four colors is formed; and a detachable optical unit including a second rotatable polygon mirror that deflects and scans a laser beam on the fifth image bearing member so as to form an electrostatic latent image on the fifth image bearing member.