An image forming apparatus includes a belt member, a driving roller arranged to support the belt member and to transmit a driving force to the belt member, a plurality of image forming units arranged to respectively form a plurality of color images on the belt member or on a recording material borne by the belt member in such a way as to have the plurality of color images superposed on top of one another, a measuring sensor for measuring a length of time required for the belt member to move a predetermined distance, and a control circuit for controlling, on the basis of the length of time measured by the measuring sensor, the timing of start of forming each of the plurality of color images by each of the plurality of image forming units on the belt member or on the recording material borne by the belt member.