An image forming apparatus capable of always performing image registration adjustment with a high accuracy and realizing a higher image quality without lowering the productivity and deteriorating a cleaning member. A first conveying speed of a sheet being conveyed on a conveying path is changed to a second conveying speed at a predetermined position to thereby perform registration of an image on an image bearing member with the sheet, at a position of the image being transferred to the sheet. Detecting an registration-use image on the image bearing member causes a first reference signal to be generated, and detecting a top end of the sheet conveyed on the conveying path causes a second reference signal to be generated. The predetermined position is determined on the basis of the first reference signal when the registration-use image sensor has successfully generated the first reference signal, and the predetermined position is determined on the basis of the second reference signal when the registration-use image sensor does not successfully generate the first reference signal.