A semiconductor laser includes an active layer having a light-emitting region of striped structure. A cladding layer is formed on the active layer. Another cladding layer is formed under the active layer. The active layer is interposed between the cladding layer and the another cladding layer. Facets are formed on both ends of a stripe of the light-emitting region in which a part of light emitted radiated, and the remaining light is reflected and amplified. The semiconductor laser includes a means for having a minimum value of a coherence in a range between 2 mW and 7 mW of output of a light radiated from one of the facets which is a light output, and for preventing a phenomenon in which the output is reduced in a range at most 25 mW in accordance with an increase of the current, varied by a variation of a current applied to the active layer.