In an image forming apparatus, a transfer material transport guide is disposed in an optical sensor unit to support a color deviation sensor, which is disposed near an intermediate transfer belt. By changing the position of the optical sensor unit, the grounding path of the transfer material transport guide can be changed, and both transfer bias leak prevention for the transfer material transport guide and electrostatic breakdown prevention for the color deviation sensor can be implemented. As a result, the apparatus can be downsized, and when the intermediate transfer belt is replaced, the optical sensor unit can be moved to create a space necessary for the replacement.