A method of producing a semiconductor optical sensor element includes the steps of: forming an oxide film on a silicon carbide substrate; forming a gate electrode layer on the oxide film; patterning the gate electrode layer to form a gate electrode; and processing thermally the gate electrode layer or the gate electrode under an oxidation environment. Further, the gate electrode layer or the gate electrode is thermally processed under the oxidation environment at a temperature between 750° C. and 900° C.