PURPOSE: To provide a quantum well semiconductor laser wherein the injection efficiency of a carrier into an active layer and the temperature characteristics are high, and the series resistance is low.

CONSTITUTION: On an N-type GaAs substrate 1, the following are laminated and formed in order; an N-type light confinement layer 15, an InGaAs strained quantum well active layer 7, a P-type light confinement layer 16, a P-InGaP clad layer 11, and a P-GaAs cap layer 12. The right side and the left side of a semiconductor laser are etched and eliminated in the cap layer 12 and a part of the clad layer 11, and a polyimide film 17 is formed in the eliminated part. In the N-type light confinement layer 15 and the P-type light confinement layer 16, InGaAsP layers different in valence band energy are laminated and formed to constitute a stepwise energy state, and the P-type light confinement layer 16 is made thicker than the N-type light confinement layer 15.