PURPOSE: To reduce speckles generated on surfaces of a mask and a wafer by making a luminous flux having a high coherence incident on the incidence surface of a fiber member where plural fibers different in refractive index are bundled.

CONSTITUTION: A laser light has the diameter expanded by an optical system 13 like a beam expander and is made incident on an optical member 15 which is provided for reduction of coherence. The optical member 15 consists of the fiber member where many fibers 15' different in internal refractive index are bundled. Luminous fluxes discharged from the optical member 15 pass a lens array 16 consisting of plural minute lenses and are collected into one again by an illuminating system 14 consisting of a condenser lens or the like and illuminate a mask 2. The lens array 16 becomes the secondary light source surface of the illuminating system 14. Thus, individual luminous fluxes do not interfere in one another on the surface of the mask 2 to prevent the generation of speckles.