PURPOSE: To return a vacuum valve quickly to an original state after the vacuum valve driven by a vacuum was driven.

CONSTITUTION: When the inside of a liquid-level detecting pipe section 56 is decompressed suddenly, pressure in a pressure control chamber 102 is made to escape quickly through the clearance of a vibration-damping diaphragm 59 and a support section 112. The pressure control chamber 102 is brought to a communicated state with an upper pressure chamber 81 by a communicating hole 103, and the inside of the upper pressure chamber 81 is also decompressed rapidly by quickly decompressing the inside of the pressure control chamber 102. Accordingly, the elastic deformation of a liquid-level detecting diaphragm 60 is removed rapidly, thus eliminating the pushing of a detecting valve 68 by a plunger 65.