PURPOSE: To shorten the time for quenching and secure the roundness at the time of honing by performing laser quenching to the bore surface of a cylinder block in such a procedure as processing spirally up and down throughout the bore.

CONSTITUTION: A laser beam irradiating device is to apply laser quenching in the form of strings 12 to the surface of the bore 2 of cylinder block 1 made of cast iron with stratified graphite, wherein the string is formed spirally up and down throughout the bore 2. Thereby irradiation of laser beam can be scanned continuously to lead to shortening of the quenching time, as well as uniform provision of quenching in the whole area will ensure that the following honing gives a good roundness and cylindricity.