An optical scanning apparatus and an image forming apparatus using the same, overcoming spot rotation due to scanning line curvature and wavefront aberration deterioration, including an incident optical system for guiding beam emitted from a light source to a deflector, and an imaging optical system for forming image of the beam deflected by the deflector on a scanning surface. In sub-scanning section, the beam enters the deflecting surface obliquely to plane perpendicular to a deflector axis. Each of incident and exit surfaces of an imaging optical element is a surface in which a tilt angle of sagittal line changes from on-axis toward off-axis in sub-scanning direction, the tilt angle indicating gradient of normal to sagittal line on meridian line with respect to main scanning section. The incident and exit surfaces each have the same sign for a difference between change rates of axial and off-axial tilt angles of sagittal line.