Provided is a III-nitride semiconductor laser diode capable of lasing to emit light of not less than 500 nm with use of a semipolar plane. Since an active layer 29 is provided so as to generate light at the wavelength of not less than 500 nm, the wavelength of light to be confined into a core semiconductor region 19 is a long wavelength. A first optical guide layer 27 is provided with a two-layer structure, and a second optical guide layer 31 is provided with a two-layer structure. A material of a cladding layer 21 comprised of at least either of AlGaN and InAlGaN is different from the III-nitride semiconductor, and the thickness D15 of a first epitaxial semiconductor region 15 is larger than the thickness D19 of the core semiconductor region 19; however, the misfit dislocation densities at first to third interfaces J1, J2 and J3 are not more than 1×106 cm−1, thereby preventing lattice relaxation from occurring in the semiconductor layers at these interfaces J1, J2 and J3 because of the c-plane that acts as a slip plane.