A semiconductor laser diode comprises: an n-type GaAs substrate; and a first laser diode structure having a first n-type cladding layer, a first active layer including a quantum well layer, a first p-type cladding layer on the first active layer, a p-type signal layer on the first p-type cladding layer and which has the same constituent elements as those of the first p-type cladding layer, and a p-type ridge waveguide in a stripe mesa-like shape on the signal layer, which has the same constituent elements as those of the signal layer, and in which composition ratios of two constituent elements in a complementary relation of constituent elements are different from those composition ratios of the signal layer.