To provide a reflective optical sensor capable of reducing consumption of a non-contributing toner and of improving attachment accuracy.

Reflective optical sensors (2245a, 2245b, 2245c) have main bodies (Opta, Optb, Optc) and γ rotary drive units (Rma, Rmb, Rmc). Each of the main bodies has an irradiation system including eleven light emission units disposed at equal intervals along the principal direction, and a light receiving system including eleven light receiving units. Each of the γ rotary drive units has a rotary table, a gear and a stepping motor and rotates the main body around an axis which goes through the center of the main body and is parallel to the Z-axis. As a result, the main body can be rotated within a plane parallel to the surface of an intermediate transfer belt 2040 which faces the main body, and the direction of a light spot array on the intermediate transfer belt can be easily and accurately made parallel to the principal direction.