A method for laser beam machining of a workpiece in which a laser beam is focused by an objective, into or onto the workpiece having a boundary surface, to produce a machining effect by a two-photon process, and the position of the focal point with respect to the workpiece is shifted. To obtain a reference for the position of the focal point, an image of a luminating modulation object is projected through the objective onto the workpiece into the focal plane or so as to intersect it. Reflections of the image occurring at the boundary surface are imaged into an autofocus image plane, and are detected by a camera. The camera image plane either intersects the autofocus image plane when the image of the illuminating modulation object lies in the focal plane, or lies in the autofocus image plane when the image of the modulation object intersects the focal plane.