PURPOSE: To reduce loss and remove effects of vibration and ambient temperature by placing a plain-convex lens which has an appropriate curvature in front of at least one reflector of a photo-resonator.

CONSTITUTION: In a laser beam shaping device, a plano-convex lens 12 which has an appropriate curvature is placed at a specified position in front of one reflector (rear mirror 10) of two reflectors, a rear mirror 10 and a front mirror 11, of a photo-resonator. In the plano-convex lens 12, the surface of the plain 12a is non-reflection coated to prevent reflection of laser beam and the convex surface 12b has a spherical curvature of radius R. The plano-convex lens 12 is precisely placed to be separated a distance (d) from the top of the convex surface 12b to the mirror surface 10a of the rear mirror 10. The position of the lens is also selected so that the phases of the reflected light on the convex surface and the transmitted light through the convex surface and reflected by the reflector coincide with each other at plural positions.