A laser processing apparatus for irradiating a planar object to be processed with processing laser light while locating a converging point within the object, so as to form a modified region to become a cutting start point within the object along a line to cut the object, the apparatus comprising: a mount table for mounting the object; a processing laser light source for emitting the processing laser light as linearly polarized light; a measuring laser light source for emitting measuring laser light for irradiating the object; characterized by a first half-wave plate for changing a polarization direction of the processing laser light emitted from the processing laser light source; a polarizing plate for splitting the processing laser light transmitted through the first half-wave plate into a processing laser light component having a polarization direction in a predetermined direction and a processing laser light component having a polarization direction in a direction intersecting the predetermined direction; a second half-wave plate for changing the polarization direction of the processing laser light component, split by the polarizing plate, having the polarization direction in the direction intersecting the predetermined direction into the predetermined direction; a first converging lens for converging the processing laser light component, split by the polarizing plate or transmitted through the second half-wave plate, having the polarization direction in the predetermined direction and the measuring laser light emitted from the measuring laser light source toward the object; a second converging lens, arranged in parallel with the first converging lens along the predetermined direction, for converging the processing laser light component, transmitted through the second half-wave plate or split by the polarizing plate, having the polarization direction in the predetermined direction toward the object; control means for detecting a reflected light component of the measuring laser light reflected by a laser light irradiation surface irradiated with the measuring laser light in the object so as to regulate a distance between the laser light irradiation surface and the second converging lens such that a converging point of the processing laser light converged by the second converging lens is located at a predetermined position with reference to the laser light irradiation surface; and moving means for relatively moving at least one of a pair of the first and second converging lenses and the mount table along the line to cut in a state where the predetermined direction substantially coincides with the line to cut. a mount table for mounting the object; a processing laser light source for emitting the processing laser light as linearly polarized light; a measuring laser light source for emitting measuring laser light for irradiating the object; characterized by a first half-wave plate for changing a polarization direction of the processing laser light emitted from the processing laser light source; a polarizing plate for splitting the processing laser light transmitted through the first half-wave plate into a processing laser light component having a polarization direction in a predetermined direction and a processing laser light component having a polarization direction in a direction intersecting the predetermined direction; a second half-wave plate for changing the polarization direction of the processing laser light component, split by the polarizing plate, having the polarization direction in the direction intersecting the predetermined direction into the predetermined direction; a first converging lens for converging the processing laser light component, split by the polarizing plate or transmitted through the second half-wave plate, having the polarization direction in the predetermined direction and the measuring laser light emitted from the measuring laser light source toward the object; a second converging lens, arranged in parallel with the first converging lens along the predetermined direction, for converging the processing laser light component, transmitted through the second half-wave plate or split by the polarizing plate, having the polarization direction in the predetermined direction toward the object; control means for detecting a reflected light component of the measuring laser light reflected by a laser light irradiation surface irradiated with the measuring laser light in the object so as to regulate a distance between the laser light irradiation surface and the second converging lens such that a converging point of the processing laser light converged by the second converging lens is located at a predetermined position with reference to the laser light irradiation surface; and moving means for relatively moving at least one of a pair of the first and second converging lenses and the mount table along the line to cut in a state where the predetermined direction substantially coincides with the line to cut.