A method for producing a thermoelectron emission source for an electron gun used in an electron beam writing apparatus, the thermoelectron emission source producing method comprising, preparing a first material that emits a thermoelectron, coating the first material with a second material having a work function larger than that of the first material, exposing the first material from part of the second material by machine processing, and decreasing a diameter of the exposed portion of the first material by heating treatment when the diameter of the exposed portion is larger than a predetermined diameter value.