In an image forming apparatus, a patch image is formed on the surface of a belt-like image support for supporting a toner image on the surface and is detected with a patch sensor placed facing a placing part on a roller for stretching the belt-like image support. An angle adjustment mechanism for adjusting the attachment angle of the patch sensor rotates the patch sensor with an intersection point of an optical axis of the patch sensor and a sensing face as the center for adjusting the attachment angle of the patch sensor. The patch sensor is attached to one member forming a part of a support frame of the image support and the attachment angle of the member is adjusted, whereby the attachment angle of the patch sensor is adjusted.