Lowering of the airtightness of the reaction chamber of the appartus due to thermal degradation of the vacuum seal of the connection between the reaction chamber and a manifold is prevented, and particles produced from members of the apparatus due to the heating of the apparatus is prevented from contaminating the inside of the chamber. The wall of the reaction chamber is extended to the location sufficiently apart from the lower end of the heater around the reaction chamber. An exhaust port is provided in the wall to exhaust the reaction chamber.
