PURPOSE: To improve the utilizing efficiency of light and reduce the fluctuation of luminance, by condensing light radiated from a light source onto a rotary regular polyhedron mirror by means of a condensing reflecting mirror and projecting the optical beam reflected and expanded in the shape of a fan upon spherical mirrors.

CONSTITUTION: Light from a light source 1 of a short-arc type mercury-arc lamp is changed to spot-like parallel light through a condensing reflecting mirror 2 having an elliptic cross section and projected upon a rotary regular polyhedron mirror 4 through plane mirrors 8 and an aspherical mirror 3. The polyhedron mirror 4 having a truncated pyramid-like shape is rotated by a motor M at a prescribed speed and light repeatedly scanned in a prescribed angle range in the from of a fan by the rotation of the mirror 4 becomes an arc-like slit of 1W4mm in width while it is repeatedly reflected by plural spherical mirrors 7 and plane mirrors 8. The arc-like light passing through a photomask 9 is repeatedly reflected among a trapezoidal mirror 10, concave mirror 11, and convex mirror 12 and forms a photomask pattern on a wafer 13 which is a body to be irradiated. By simultaneously moving the photomask 9 and wafer 13 in parallel with each other, the whole surface of a previously designated wafer can be exposed to the arc-like light.