PURPOSE: To remove heat well from a condenser at the time of isolation by discharging noncondensing gases well.

CONSTITUTION: A variable exhaust pipe 13 surrounds the terminal part of a fixed exhaust pipe 10 and plates 14, 14 for fixing a bellows 13 are fixed to the outer face of the fixed exhaust pipe 10 and to the inner face of the variable exhaust pipe 13. A bellows 15 is then fixed between the fixing plates 14, 14 and a doughnut type float 12 is fixed to the outer peripheral face of the variable exhaust pipe 13 at the upper part thereof. To the bellows fixing plate 14, a shock absorbing member 16 is attached.