PURPOSE: To carry out accurate machining of a specific shape on a work piece by converging a laser beam with a lens having a specific shape.

CONSTITUTION: When a silicon nitride sintered ceramic plate 2 is irradiated with a KrF excimer laser beam (248nm wave length, 30nsec pulse width) through a conical lens 1a made of a molten quartz, a ring with an inner diameter of 50μm and a width of 200μm is drawn. By a 500 pulse irradiation with an energy of 4J/cm2, a groove with a depth of 45μm is formed. By this method, a laser abrasion is activated only in a ring shaped area, and an annular grooving can be performed without moving a sample 2. The annular diameter is adjusted by varying a distance between the lens 1a and the sample 2, and the annular width is adjusted by varying the apex angle α or the refractive index of the conical lens 1a. In addition, if the shape of the lens is made cylindrical, a linear spot is obtained; and if a triangular prism, a parallel spot is obtained.