A method of fabricating a semiconductor device having high output power and excellent long-term reliability by preventing thermal adverse influence exerted at the time of window structure formation is provided. The method comprises a 1st step of forming predetermined semiconductor layers 2 to 9 containing at least an active layer 4b consisting of a quantum well active layer on a semiconductor substrate 1; a 2nd step of forming a first dielectric film 10 on a first portion of the surface of the semiconductor layers 2 to 9; a 3rd step of forming a second dielectric film 12 made of the same material as that of the first dielectric film 10 and having a density lower than that of the first dielectric film 10 on a second portion of the surface of the semiconductor layers 2 to 9; and a 4th step of heat-treating a multilayer body containing the semiconductor layers 2 to 9, the first dielectric film 10, and the second dielectric film 12 to disorder the quantum well layer below the second dielectric film 12.