To enable simultaneous treatment of a plurality of sheets, instead of leaf-treatment, wherein sheets are treated one by one, improve throughput, and realize cost reduction, by arranging a silicon substrate in a vacuum vessel, exposing the substrate to hydrogenated germanium gas and heating the substrate at the same time.

A silicon substrate 3 is heated at 800°C, and at the same time, germane gas is introduced into a vacuum vessel 1 through a flow rate adjusting valve 4. During 20 minutes when firstly germane is introduced, germane is thermally decomposed on the surface of the silicon substrate 3, and germanium is deposited on the surface. Deposited germanium reacts with SiO2 as a silicon oxide film on the substrate surface, and oxygen atoms O in the silicon oxide film evaporate as GeO. During the treatment, the silicon oxide film on the surface is eliminated completely, and a small amount of germanium is left on the surface. This germanium is eliminated by heating a steel plate in a vacuum for 20 minutes.