PURPOSE: To obtain a pin hole whose diameter is not varied by the heat of a condensing light beam, at a low cost and exactly, by forming a pin hole a space frequency filter by adjacently placing plural shielding substances whose section is circular, such as a spherical body, a cylindrical body, etc.

CONSTITUTION: Plural shielding substances 1 whose section is circular are prepared by at least 3 pieces or more, and a shielding substance group A is formed by placing them on a prescribed virtual plane, in a state that their circumferential ends adjacently contact with each other. By each circumferential surface part 1a between each adjacent point of each shielding substance 1, a gap space S is formed so as to surround, and it becomes a pin hole. 2 or more shielding substance groups are provided, and when they are provided by mutually turning by a prescribed angle, the gap space becomes smaller and approaches a true circle. Also, it is possible to bring a shape of the gap space close to a true circle by increasing the number of shielding substances for constituting one shielding substance group.