To prevent penetration of condensable gas by monitoring the sealing quality of a furnace door and reliably measuring a through flow of gas.

A flange-shaped projecting portion 12 can hermetically butt on an annular end face of a furnace casing 2, and a lid 18 covers a central cap-shaped member 16 of the hat-shaped furnace door 6 extending into the furnace casing 2 in a closed state, and is slidably held and guided on the furnace door 6. The outside diameter of the lid 18 is set larger than that of the cap- shaped member 16 of the furnace door 6 and smaller than the inside diameter of the furnace casing 2. A radially outer end face portion of the lid 18 corresponds to a step part 21 disposed on an inner wall 22 of the furnace casing 2 to form an annular sealing surface, and butts on the step part 21 to limit an annular cylindrical chamber 24. The chamber 24 is formed by the substantially cylindrical member 16 of the furnace door 2, the flange-shaped projecting portion 12 and a part of the inner wall 22 of the furnace casing 2.