To clean semiconductor manufacturing equipment in a short time, by oxidizing inside of a heating furnace of a semiconductor manufacturing equipment so as to oxidize impurities existing on surface parts in the heating furnace, and reducing impurity diffusion into a substrate to which film forming process is to be performed in the heating furnace.

Many silicon substrates W supported at a horizontal position by a support 6 stood in a heat insulating tower are inserted into an inner tube 4 of a heating furnace of semiconductor manufacturing equipment from the bottom, and the furnace is tightly closed with a gate valve. Then, O2(oxygen) gas is introduced into the heating furnace to thermally oxidize the substrates at a temperature of 900°C, O atoms are permitted to enter the P-Si bonding in a P added Si film 20, and on a surface side part 21 where the P added Si film 20 is oxidized, P is oxidized into POx(phosphorus oxide) and Si into SiOy or SiOz (both are silicon oxides). By the oxidation of the surface side part 21, impurities are oxidized and impurity diffusion within the silicon substrates W to which film forming process is performed in the heating furnace is reduced.