PURPOSE: To manufacture a semiconductor laser having a highly reliable quantum well thin-wire structure at high reproducibility by constituting an active layer on the top of a layer which is epitaxially grown on a mesa projection.

CONSTITUTION: After an epitaxially grown layer 10 having a triangular cross section surrounded by {111} B crystal faces is formed on a mesa projection 2 elongated in <011> crystal axis direction and the top section 10a of the layer 10 is formed, the epitaxial growth is stopped and the partial pressure of compound semiconductor layer constituting elements contained in the gaseous starting material for epitaxial growth is lowered. Thereafter, an active layer 4 is formed on the top 10a of the layer 10 by raising the partial pressure of the elements and performing epitaxial growth on the mesa projection 2. Therefore, oxidation, mixture of impurities, etc., of the active layer can be prevented and, at the same time, a precise quantum well thin-wire structure can be surely formed with high accuracy beyond the etching accuracy.