PURPOSE: To reduce costs and space and to prevent a shift in image forming point by using optical members other than a splitting mirror in common in splitting an optical path while using the splitting mirror exclusively.

CONSTITUTION: To emit a beam of light from a point A on an original surface to the center point L of a lens 10 as if it were emitted from a point C on the original surface, the 1st splitting mirror 11 is interposed between the point A and the center point L of the lens, thereby bending the optical path. Similarly, to emit a beam of light traveling from a point B on the original surface to the point L of the lens as if it were emitted from the point C on the original surface, the 2nd splitting mirror 12 is interposed between the points B and L, bending the optical path. Consequently, an image at the point C on the original surface is split and projected on an image-forming surface and projected upon solid-stage scanning elements 7 and 8 respectively. In the figure, a mirror lens 4 consists of a lens 3 and a Dach-prism 5.