The method for producing the optical semiconductor of the present disclosure includes a mixing step of producing a mixture containing a reduction inhibitor and a niobium compound that contains at least oxygen in its composition; a nitriding step of nitriding the mixture by the reaction between the mixture and a nitrogen compound gas; and a washing step of isolating niobium oxynitride from the material obtained through the nitriding step by dissolving chemical species other than niobium oxynitride with a washing liquid. The optical semiconductor of the present disclosure substantially consists of niobium oxynitride having a crystal structure of baddeleyite and having a composition represented by the composition formula, NbON.