Provided is a method of manufacturing a semiconductor laser diode. The method includes the steps of: preparing a GaN substrate having an a-plane or m-plane GaN layer formed thereon; forming a plurality of laser diode structures on the GaN layer; etching the GaN substrate such that a cutting reference line is formed in a groove shape along the crystal surface of the a-plane or m-plane, not a main plane; and cutting the GaN substrate along the cutting reference line so as to form a mirror surface of the semiconductor laser diode, the mirror surface coinciding with the crystal surface of the a-plane or m-plane, not the main plane.