Optical scanning device and image forming apparatus using the same

An optical scanning apparatus comprising:

• light source means;
• deflecting means for scanningly deflecting a beam emitted from said light source means;
• an imaging optical system including an imaging optical element for imaging the beam deflected by a deflection surface of said deflecting means on a surface to be scanned;
• wherein a beam emitted from said light source means is incident on the deflection surface of said deflecting means at an angle relative to an optical axis of said imaging optical system with respect to a sub-scan direction, and a principal ray of the beam deflected by the deflection surface of said deflecting means passes through other than the optical axis of said imaging optical system in the sub-scan section, said imaging optical element includes a sagittal aspherical surface amount changing surface in which a sagittal aspherical amount ΔXz increases along a main scan direction away from the optical axis of said imaging optical system to an extremal value and then decreases,
• wherein the sagittal aspherical amount ΔXz of said sagittal aspherical amount changing surface changes along the sub-scan direction away from the optical axis of said imaging optical system, and said sagittal aspherical amount changing surface has a sagittal radius of curvature which changes along a main scan direction, and
• wherein the sagittal aspherical amount ΔXz of said sagittal aspherical amount changing surface is ΔXz=0 on the optical axis of said imaging optical system.