Before applying a processing laser beam to a surface of a substrate through a film of liquid, distance M2 between a reference point on an axis of a first laser displacement meter and the surface of the substrate is measured to correct distance M1 between a lower end of an optical unit and the surface of the substrate, on the basis of distance M2, the processing laser beam is applied to the surface of the substrate, thereby cutting and removing a part of the surface of the substrate, and a depth to which the surface of the substrate has been cut and removed with the processing laser beam is measured by a second laser displacement meter.