To provide a laser beam machining method in which a uniform modified layer can be formed in a workpiece without reference to the state of an irradiated surface of the workpiece irradiated with a laser beam.

A laser beam machining method includes: a reflectivity detecting step of detecting reflectivity of a laser beam to be emitted to an irradiated surface irradiated with a laser beam, of a workpiece 1; an antireflective film forming step of forming an antireflective film 5 on the irradiated surface of the workpiece 1 based upon the detected reflectivity so that the irradiated surface has reflectivity lower than predetermined reflectivity after executing the reflectivity detecting step; and a laser beam machining step of forming a modified layer in the workpiece 1 by irradiating the irradiated surface (a surface of the antireflective film 5) of the workpiece 1 with a laser beam L with a wavelength having permeability after executing the antireflective film forming step. Consequently, the uniform modified layer can be formed since the inside of the workpiece 1 is irradiated with the laser beam with the surface of the antireflective film 5 as the irradiated surface.