A first mirror (5) that can be located at a reflection position (I) at which an optical path is blocked and a laser beam (a) is reflected, and a second mirror (6) that reflects the laser beam (a) which is reflected by the first mirror (5) are disposed between the condenser lens (2) and the object to be irradiated (4). The first mirror (5) is located at the reflection position (I) so that the laser beam (a) that is transmitted through the condenser lens (2) is sequentially reflected by the first and second mirrors (5 and 6), and an intensity of the laser beam (a) that is again reflected by the first mirror (5) is made to coincide with an intensity of the laser beam (a) that is reflected from the object to be irradiated (4), and the condenser lens (2) is heated in the same manner that the processing laser beam (a) is transmitted through the condenser lens (2) and irradiated on the object to be irradiated (4).