To provide a laser processing apparatus which can converge a laser beam for processing an object to be processed at a position as close as possible to a predetermined position.This laser processing apparatus is one for irradiating an object to be processed with a processing laser beam while positioning a converging point within the object so as to form a modified region caused by multiphoton absorption within the object along a line along which, the object should be cut in the object, and comprises an objective lens unit 5 including a processing objective lens 42 which converges the processing laser beam and a rangefinding laser beam for measuring a displacement of a main surface of the object on an identical axis, a light-receiving part 45 which receives a reflected light beam of the rangefinding laser beam on the main surface, and an actuator 43 which holds the processing objective lens 42 according to the received reflected light beam such that the lens 42 is capable of freely advancing and retracting with respect to the main surface; and a housing 21 to which a laser unit for emitting the processing laser beam is attached. The objective lens unit 5 is detachably attached to the housing 21.