By a first image forming optical system (42a to 42d), a plurality of light beams from light sources (41a to 41d) form linear images on a common deflecting surface (46) of an optical deflector (44). Light beams reflected off the optical deflector (44) are reflected by a plurality of curved surface mirrors (45a to 45d) and are allowed to scan over photosensitive members (4a to 4d), respectively. The plurality of curved surface mirrors (45a to 45d) are disposed on the same side with respect to a plane that includes a normal line at a center of the deflecting surface (46) and is parallel to a main scanning direction. Further, curved surfaces of the plurality of curved surface mirrors (45a to 45d) vary in shape. Thus, a tandem type color image forming apparatus that achieves low cost and has excellent optical performance and an optical scanning device that is used favorably in the color image forming apparatus can be provided.