The present invention provides a semiconductor laser including a lower clad layer, an active layer including at least one quantum well layer, and an upper clad layer formed in this order above a semiconductor substrate, and having a window region including a portion in which the quantum well layer in the active layer and layers adjacent to the active layer are intermixed in the vicinity of a light emitting end face perpendicular to the surface of the semiconductor substrate, in which the lower clad layer has a refractive index higher than that of the upper clad layer, and the light intensity distribution in the window region spreads more widely in the direction perpendicular to the surface of the semiconductor substrate than the light intensity distribution in the gain region, and also provides a method for fabricating such a semiconductor laser.