An optical scanning device for scanning a photoreceptor surface with a beam, said optical scanning device having: a light source for emitting the beam; an optical system for causing the beam emitted from the light source to converge; and a deflector that includes a polygon mirror with a plurality of reflecting surfaces and that deflects the beam that has passed through the optical system by rotations of the polygon mirror. Between the deflector and the photoreceptor surface, no optical system for causing the beam to converge or diverge is disposed. The optical system for causing the beam emitted from the light source to converge generates spherical aberration depending on which part of the optical system in a main-scanning direction the beam passes through. The beam enters into three or more adjacent reflecting surfaces of the polygon mirror at a time.