A method for fabricating a group-III nitride semiconductor laser device having a semi-polar surface provides a laser cavity mirror which can reduce lasing threshold current. A support plate H tilts at an angle THETA from an m-axis toward a reference plane Ab defined by a direction PR of travel of the blade 5g and an a-axis in a c-m plane while the direction PR is being orthogonal to the front surface Ha of the support plate H. The blade 5g is positioned so as to be aligned to a plane which includes an intersection P1 between the endmost scribe mark 5b1 among a plurality of scribe marks 5b and the front surface 5a of the substrate product 5 and extends along the direction PR. In the case where the angle ALPHA defined ranges either from 71 to 79 degrees or from 101 to 109 degrees, the angle THETA then ranges from 11 to 19 degrees, and thereby the reference plane Ab along the direction PR extends along the c-plane orthogonal to the c-axis.