An image forming apparatus includes a main assembly including an image forming portion; a rotatable member capable of feeding a sheet and configured to rotate; and a detecting unit including an interrelating member configured to rotate in interrelation with rotation of the rotatable member, a detecting member configured to detect rotation of the interrelating member, and a holding member configured to integrally hold the interrelating member and the detecting member. The interrelating member, the detecting member and the holding member are integrally assembled into a unit. The detecting unit is detachably mountable to the main assembly.