the laser light having penetrated to any portion other than the stripe portion so that the lasing at the lowest order mode is easy now description will be made of the method of fabricating the mesa stripe structure semiconductor injection laser according to this invention by the continuous liquid phase epitaxial growth the sn doped n ga 1 x al x as for example x 3 layer 3 non doped gaas active layer 4 ge doped p ga 1 x al x as layer 5 and ge doped p gaas layer 6 are successively grown on the n gaas substrate 2 subsequently via a photo resist process an sio 2 film is deposited on the surface of the p gaas layer 6 to become the part of the current injection region in the form of a stripe by the chemical vapor deposition using the sio 2 film as a mask both the outer sides of the current injection region are removed down to a predetermined depth by the chemical etching and the corresponding parts of the p ga 1 x al x as layer 5 are thinned down to a predetermined thickness as an etchant at this time there is an hf series an hcl series an h 2 so 4 series an h 3 po 4 series or the like when as an example an etchant in which h 3 po 4 h 2 o 2 and ch 3 oh are mixed in proportions of 1 1 3 is employed.