PURPOSE: To erect a sealing structure resistant to high vacuum state even if a metallic thin sheet is used by a method wherein the gap between a reaction tube and an outer flange part and a manifold trench part are evacuated at reduced pressure.

CONSTITUTION: A flange part 14 is circularly formed on an opposite surface to another flange part 11 on the upper end part of a manifold 13 while a circular trench part 15 is formed on the surface of the flange part 14. On the other hand, an outer flange 16 as a fixture for connection-fix an outside reaction tube 1 to the manifold 13 is provided on the outer periphery of the flange part 11 of the reaction tube 2 so as to form a circular gap 18 between the outer flange 16 and the flange parts 11, 14. Through these procedures, exhaust pipes 23, 24 connected by the circular trench 15 and the gap 18 can be connected by exhaust means 60 and connectors 61, 62 thereby enabling the circular trench part 15 and the gap pipe 18 to be evacuated at reduced pressure. Accordingly, the inside of the reaction tube 2 can be retained in high vacuum state.