PURPOSE: To effectively illuminate the irradiation region of the desired shape in a light integrator by a method wherein the light integrator is constituted in a structure that the segment lenses, each having the section of the same shape as that of the desired irradiation region are densely arranged in the planes vertical to the optical axis to form the irradiation region in the desired shape directly on the surface to be irradiated.

CONSTITUTION: A light integrator 3 has the sections of the X and Y surfaces which are vertical to the optical axis, in the same shape as that of the desired irradiation region. That is, the light integrator 3 is constituted in a structure that the segment lenses, each being formed into a circular arc shpae or a fan shape, are densely arranged in the X and Y planes. The lights emitted from a superhigh pressure mercury-arc lamp 1 are focussed on the light integrator 3 by an oval mirror 2, the lights emitted from each point of the light integrator 3 turn into respectively a parallel ray by the action of a collimator lens 4 and the parallel rays irradiate the surface of a mask 5. That is, a multiluminous flux Kohler illumination results in being projected on the mask 5 and a circular arc-shaped or a fan-shaped irradiation region can be formed on the position of the mask 5.