An inner drum exposure apparatus is disclosed in which a linearly polarized light beam emitted from a light source is converted to circularly polarized light. This in turn is passed through a single-axis crystal optical element so as to be split into equal light quantities of an ordinary ray and an extraordinary ray in a sub-scanning direction. These rays are subsequently reflected at a reflective mirror surface of a scanning unit so as to be focussed at a beam spot which is distributed on a recording medium in an approximately rectangular configuration with respect to the sub-scanning direction and has steep edge portions, thereby performing exposure processing.