To provide a projection optical system and an image forming device by which a viewing angle can be made large and high resolution is realized without increasing the number of lenses.

An image forming lens part 4 which is provided with the aspherical-surface lenses 41 and 42 having a 1st shape for correcting the aberration of plural laser beams in the 1st direction and a 2nd shape for correcting the aberration in a 2nd direction orthogonally crossing the 1st direction and whose object side is formed to be telecentric is arranged. Then, a diaphragm 43 is arranged between and the lens part 4 and a photoreceptor drum 3. Thus, the viewing angle can be made large and the high resolution is realized without increasing the number of lenses.