Abstract: An improved laser diode is made of a gallium nitride compound semiconductor ((Al.sub.x Ga.sub.1-x).sub.y In.sub.1-y N; 0.ltoreq.x.ltoreq.1; 0.ltoreq.x.ltoreq.1) with a double heterojunction structure having the active layer held between layers having a greater band gap. The laser diode comprises mirror surfaces formed by cleaving the multi-layered coating and the sapphire substrate in directions parallel to <0001> (c axis) of the sapphire substrate. The intermediate zinc oxide (ZnO) layer is selectively removed by wet etching with a ZnO-selective liquid etchant so as to form gaps between the sapphire substrate and the bottom-most sub-layer of the semiconductor laser element layer. The semiconductor laser element layer is cleaved with the aid of the gaps, and the resulting planes of cleavage are used as the mirror surfaces of the laser cavity.