To provide an image forming apparatus capable of performing the exposing/shielding control of a density sensor with simple device constitution, and a transfer device.

A shield disk 65c where a notch part 65c1 is formed is arranged between the density sensor 65b and a belt 61. By rotating the shield disk 65c, the exposing/shielding state of the density sensor 65b relative to the belt 61 is switched. A disk supporting shaft gear 65f1 is formed on a disk supporting shaft 65f being the rotation center axis of the shield disk 65c, and a disk driving gear 65g is rotatably supported in a sensor frame 65a so as to mesh with the disk supporting shaft gear 65f1. The disk driving gear 65g is coupled with a driving force transmitting mechanism for driving and rotating a belt driving roller 63, and the shield disk 65c is rotated any time through the disk driving gear 65g when the belt driving roller 63 is driven and rotated.