An exposure process is executed by multi-beams divided by polarizing and combining laser beams (La, Lb) modulated based on an image signal so as to be emitted by a light source side optical system by using a polarized light beam splitter, and by converting the laser beams (La, Lb) emitted as right circular polarized light and left circular polarized light into mutually orthogonal linear polarized light by using a quarter wave plate arranged on a light path of a scanning portion, and thereafter transmitting through an optical element of uniaxial crystal. An appropriate image can be formed by setting a division width of beam spots on a scanning surface set by the optical element of uniaxial crystal such that scanning unevenness of an image formed on a scanning surface is within an allowable range even when changing resolution of the inner drum exposure apparatus.