PURPOSE: To easily design and to manufacture a beam shaping optical system with a high precision by satisfying specific conditions between the extent of dissatisfaction of the sine condition of a front group of the optical system and that a rear group.

CONSTITUTION: A beam shaping optical system like an afocal optical system which converts the luminous flux having the intensity of light of regular distribution or the like to an uniform intensity of light consists of a front group G1 and a rear group G2. The extent of dissatisfaction of the sine condition of at least one both lens groups is generated to satisfy inequalities (1)W(3) between an extent S1 of dissatisfaction of the sine condition of the front group and an extent of S2 of that of the rear group. If the condition of an inequality (4) is satisfied, this optical system is more effective. Thus, the optical system is obtained which is easy to design and manufacture and converts the luminous flux having the intensity of light of Gaussian distribution to a luminous flux having a more uniform intensity of light.