PURPOSE: To modify uniformly and quickly an inner surface of a recessed part of a material to be machined by laser beam by condensing laser beam to a microscopic spot with two slanting surfaces of a prism which is freely ascendable and descendable in the recessed part of the material to be machined.

CONSTITUTION: The partly reflecting prism 2 consisting of an optically isotropic body which can ascend and descend is inserted and held with its two slanting surfaces 2a, 2b having a specified angle upwards at the inside of the recessed part 1a on the upper face of the material 1 to be machined consisting of a metallic material. When the prescribed laser beam 3 is then irradiated toward the prism 2 from the opening side of the recessed part 1a, the beam 3 is reflected with the slanting surfaces 2a, 2b and irradiated simultaneously on both side wall surfaces of the inside of the recessed part 1a. A part of the beam 3 is also penetrated the prism 2 and irradiated onto the surface of the bottom face of the inside of the recessed part 1a. A specified modified layer 4 is thus formed.