In an image forming apparatus provided with an image forming portion for forming a toner image on a rotatable image bearing member, and a detecting portion for detecting a toner image for detection formed on the image bearing member, wherein the state of the image forming portion is controlled on the basis of the result of detection of the toner image by the detecting portion, and the result of detection of the surface of the image bearing member on which the toner image is not formed by the detecting portion, the detection of the surface of the image bearing member on which the toner image is not formed by the detecting portion is effected at each substantially 1/n cycle (n being 2 or greater integer) in one revolution of the image bearing member.