A scanning optical device and color image forming apparatus use a
refraction element and diffraction element as the scanning optical element
of a scanning optical device, the powers of the refraction element and
diffraction element being set such that an imaging position deviation in
the main scanning direction caused by a wavelength variation along with
the environmental fluctuation of the scanning optical device and a imaging
position deviation caused by the refractive index variation of the
material of the scanning optical element are set to opposite directions at
the two ends of an effective image region (effective scanning region).
This reduces imaging position deviation caused by an environmental
fluctuation, and limits imaging position deviation caused by a wavelength
variation not accompanied by any environmental fluctuation. By this means
it is possible with a simple arrangement at low cost to prevent color
image misregistration between colors caused by a wavelength fluctuation or
environmental fluctuation and largely to avoid density nonuniformity
between colors.
