A method of manufacturing a nitride-based semiconductor laser diode that can minimize optical absorption on a cavity mirror plane and improve the surface roughness of the cavity mirror plane is provided. The method includes the steps of: forming on a (0001) GaN (gallium nitride) substrate having at least two masks spaced apart by a distance equal to a laser cavity length in stripes that extend along the <11-20> direction; growing an n-GaN layer on the GaN substrate between the masks so that two (1-100) edges of the n-GaN layer are thicker than the remaining regions thereof; sequentially stacking an n-clad layer, an active layer, and a p-clad layer on the n-GaN layer to form an edge-emitting laser cavity structure in which laser light generated in the active layer passes through a region of the n-clad layer aligned laterally with the active layer and is output; and etching a (1-100) plane of the laser cavity structure to form a cavity mirror plane.