PURPOSE: To form a quenched layer having a depth less than a predetermined value in the surface layer of a bore in a cylinder block made of piece-like graphite cast iron to enhance the durability of the cylinder head with no thermal cracks, by irradiating a laser beam to a suitable part in the bore surface of the cylinder block.

CONSTITUTION: A laser beam irradiating device 1 composed of a laser generating head 2, reflecting mirrors 3, 4, a condensing lens 5, etc. is confronted to the inner diameter section of a bore 7 in a cylinder block 6, and disposed rotatably about the cylindrical axis of the bore 7 and movably in the vertical direction. Meanwhile, a quenched part 9 formed in the inner peripheral surface of the bore 7 is provided in a part where abrasion due to contact with respect to a piston ring is fierce, such as, for example, the area ranging from a position in the vicinity of the top dead center of a piston to a suitable descending level. With this arrangement, a quenched part having a depth up to a predetermined depth of 0.35mm may be formed in the outermost layer of the bore with no thermal cracks being generated, by changing the diameter of spot section of a laser beam 8 and the scanning speed of the beam.