To make clear the ideal structure for the image forming system between an aperture specifying a lighting area and a reticle and, at the same time, to provide an optical illumination system having the image forming system.

An aligner has the optical illumination system which illuminates a reflection type mask with the light from a light source, and a projection optical system which projects the pattern of the reflection type mask upon a substrate. The optical illumination system has a field stop which specifies the lighting area of the reflection type mask and the image forming system which leads the light from the aperture of the field stop to the mask. The image forming system is a coaxial optical system, and the principal ray of the image forming system on the mask side has an angle of inclination to the co-axis of the system. The angle of inclination is almost equal to the angle between the principal ray of the projection optical system on the mask side and the normal line to the surface of the mask.