PURPOSE: To apply illumination on the circumferential part only of the position of irradiation of a laser beam and to conduct a trimming operation easily and accurately by a method wherein the irradiation angle of the illumination light against an irradiated object and the irradiation angle of the laser beam are almost made equal.

CONSTITUTION: An intense heat is generated on the irradiation part of a laser beam 14, a part of a resistor 32 is disconnected by vaporization, and the resistance value of the resistor 32 changes according to the above-mentioned cut length. Accordingly, a desired resistance value can be obtained by adjusting the length of cutting. Said cutting length is adjusted by changing the angles of the reflecting mirror 22a of a galvanometer 22 and the reflecting mirror 24a of a galvanometer 24. The illumination light 17 emitted from an illuminator 4 is reflected by the reflecting mirror 22a of the galvanometer 22, which is an irradiation position moving means, and the reflecting mirror 24a of the galvanometer 24, and it is projected to the resistor 32 on a substrate and an illumination light irradiation spot 40 is formed. To be more precise, the irradiation position of a laser beam 14 is moved by the reflecting mirror 22a of the galvanometer 22 and the reflecting mirror 24a of the galvanometer 24, the photographing place of moving picture camera 2 is determined by the above-mentioned reflection mirrors, and besides, the irradiation position of an illumination light 17 is also controlled by them.