In a semiconductor laser element, a lower cladding layer of AlGaInP of the first conductive type, a lower optical waveguide layer of AlGaInP, a quantum-well active layer of InGaP, an upper optical waveguide layer of AlGaInP, and an upper cladding layer of AlGaInP of the second conductive type are formed in this order on a substrate of GaAs of the first conductive type. The degree of mismatch .DELTA.a/a with the substrate and the thickness dw of the quantum-well active layer satisfy the conditions, -0.6%.ltoreq..DELTA.a/a.ltoreq.-0.3% and 10 nm.ltoreq.dw.ltoreq.20 nm. In addition, the resonator length Lc and the reflectances Rf and Rr of the opposite end facets satisfy the conditions, Lc.gtoreq.400 .mu.m and Rf.times.Rr.gtoreq.0.5.