A substrate processing method for forming a space extending along a predetermined line in a silicon substrate includes a first step of converging a laser light which is an elliptically-polarized light having an ellipticity other than 1 at the substrate so as to form a plurality of modified spots within the substrate along the line and produce a modified region including the modified spots, and a second step of anisotropically etching the substrate so as to advance an etching selectively along the modified region and form the space in the substrate. In the first step, the light is converged at the substrate such that a moving direction of the light with respect to the substrate and a direction of polarization of the light form an angle of 45° or greater therebetween, and the modified spots are made align in one row along the line.