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 concave mirrors of a reflection mirror which is freely ascendable and descendable in the recessed part of the material to be machined.
CONSTITUTION: The reflection mirror 2 consisting of two concave mirrors 2a, 2b formed in an angular shape having a specified angle is inserted into the recessed part 1a on the upper face of the material 1 to be machined consisting of a metallic material and is held adequately freely ascendable and descendable. When the prescribed laser beam 3 is then irradiated toward the mirror 2 from the opening side of the recessed part 1a, the beam 3 is irradiated simultaneously on both side wall surfaces of the inside of the recessed part 1a with reflection by the mirrors 2a, 2b, and the specified modified layer 4 is formed.