PURPOSE: To eliminate the optical damage of an optical element by a method wherein a homogenizer which makes the brightness distribution of an illumination light flux uniform and a condensing optical system with which an object to be irradiated is irradiated are constituted of a zone plate.

CONSTITUTION: A light flux 2 which is radiated form a laser light-source generation device 1 is reflected by a reflector 3; it is incident on a homogenizer 4 as an optical element which forms the more uniform brightness distribution of a plane to be irradiated. The light flux which is divided by the homogenizer 4 forms a secondary light source 5 on the focal plane of the homogenizer, and illuminates, by using an installed condensing lens 6, a plane 7 to be irradiated where a mask is installed. The homogenizer 4 and the condensing lens 6 are formed of a zone plate. Thereby, it is possible to constitute an illumination optical system wherein the loss of the quantity of light is small with reference to a short-wavelength light source and the optical damage of an optical component is not caused.