NOVELTY - A reaction container (100) made of carbon/graphite consisting of carbon containing material has a joint for carrying out gas phase reaction of chlorosilane and hydrogen, and a gas supply device (500) supplies inert gas containing argon gas to the space formed between reaction container and an outer cylinder container (300). An inert gas pressure regulator (600) adjusts the supply amount rate of inert gas so that the external atmospheric pressure of the space becomes higher than the internal atmospheric pressure in the reaction container.    USE - Reaction apparatus used in production of chlorosilane (claimed) such as trichlorosilane used in semiconductor and solar cell.    ADVANTAGE - The thinning, embrittlement, or decline in the operation efficiency of the apparatus can be prevented by deteriorating physically and chemically. The production efficiency of chlorosilane can be improved, and the need of carrying out the temporary rest of the working of a process by repair, replacement, etc., of the reaction container or an outer cylinder container is reduced.    DETAILED DESCRIPTION - The reaction apparatus is adjusted so that external atmospheric pressure turns into 102% or more of the internal atmospheric pressure. The reaction container consists of several carbon containing materials arranged substantially coaxially. The surface of the inner side of the reaction container and the outer side is coated with a 10-500 mu m thick silicon carbide film formed using chemical vapor deposition (CVD) film. INDEPENDENT CLAIMS are included for the following:    (1) method of preventing corrosion of reaction apparatus; and    (2) production method of chlorosilane using reaction apparatus, that involves supplying a mixed gas containing chlorosilane gas and hydrogen gas in the reaction container and heating it within the reaction container with a heater. The chlorosilane gas and hydrogen gas in the mixed gas are made to react, and chlorosilane gas is restored.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic sectional view of the reaction apparatus.    Reaction container (100)    Outer cylinder container (300)    Inert gas supply device (500)    Inert gas pressure regulator (600)    Reaction apparatus (1000)