PURPOSE: To reduce the blur of an aperture image to improve the capability of a face to be illuminated, by separating concaved and convexed light shielding plates, which constitute an arc-shaped slit aperture, from each other in the direction of the optical axis in an illuminating system using spherical mirrors.

CONSTITUTION: A concaved light shielding plate 18A and a convexed light shielding plate 18B of a light shielding plate 18 constituting the arc-shaped slit aperture of the illuminating system where concave mirrors 11W13 are used to print a semiconductor are separated from each other in the direction of the optical axis and are so set that focus points S1 and S2 of respective end parts P1 and P2 are formed on the same plane K. The light of a spot light source 5 is reflected by the mirror 11 and is converged on the arc-shaped aperture. The light passing through the aperture part is reflected by mirrors 12 and 13 and is converged on the plane K. Since focus points S1 and S2 are formed on the same plane K, the blur of recessed and projecting edges is reduced very much, and the capacity of the face to be illuminated of semiconductor printing or the like is improved.