The invention provides a method for forming a selective mask on a surface of a layer of AlXGaYIn1-X-YAsZP1-Z or AlXGaYIn1-X-YNZAs1-Z (0≦X≦1, 0≦Y≦1, 0≦Z≦1), which is a method for forming a mask with a minute width suitable for microfabrication in nano-order.(1) An energy beam 4a, 4b is selectively irradiated onto a natural oxide layer 2 formed on the surface of the layer 1 of AlXGaYIn1-X-YAsZP1-Z or AlXGaYIn1-X-YNZAs1-Z. (2) Of the natural oxide layer 2, parts other than parts onto which the energy beam 4a, 4b has been irradiated is removed by heating. (3) The natural oxide layer 2 of the parts onto which the energy beam 4a, 4b has been irradiated is partially removed by heating while alternatively carrying out a rise and fall in heating temperature.