Laser irradiating method including maintaining temperature of a lens

A laser irradiating method comprising:

• emitting a laser beam from a laser oscillator;
• guiding the laser beam to a condenser lens;
• positioning a first mirror between the condenser lens and an object to be irradiated, wherein the first mirror is movable between a reflection position (I) at which an optical path between the condenser lens and the object to be irradiated is blocked, and at an open position (II) at which the optical path between the lens and the object to be irradiated is open;
• positioning a second mirror so as to be arranged perpendicularly to an optical axis of the processing laser beam that is reflected by the first mirror when located at the reflection position (I), the second mirror reflecting the processing laser beam with a given reflectivity;
• moving the first mirror to the open position (II) and irradiating the object with the processing laser beam transmitted through the condenser lens;
• and
• moving the first mirror to the reflection position (I) to block the optical path between the condenser lens and the object, wherein the first mirror reflects the processing laser beam with a given reflectivity to the second mirror, which partially reflects the processing laser beam so that the processing laser beam is again reflected by the first mirror and is transmitted through the condenser lens, thereby heating the condenser lens in the same manner as when the first mirror is located at the open position (II) and the processing laser beam is transmitted through the condenser lens and irradiated on the object.