To prevent the life of a lens member from being damaged even if the excimer laser of high output is used by constituting an afocal optical system by a first lens group having negative refracting power, a second lens group having positive refracting power and a third lens group having positive refract ing power in order from a light source-side.

The afocal optical system is constituted of the first lens group G1 having negative refracting power, the second lens group G2 having positive refracting power and the third lens group G3 having positive refracting power in order from the light source-side. Parallel luminous flux from the excimer laser light source is diverged since the first lens group G1 has negative refracting force and it is made incident on the second lens group G2. Since the second lens group G2 has positive refracting force, diverged light is changed into converged light and it is focused. Although luminous flux is made incident on the third lens group G3 having positive refracting force, the former focal position F of the third lens group G3 is matched with a converged point by the second lens group G2, and projected light from the third lens group G3 is made into parallel luminous flux.