PURPOSE: To detect gas leakage easily and accurately by checking for the movement of liquid in a U-shaped tube connected to a communicating hole provided the undersurface of a fixed lid airtightly inserted into a mouth section of a draft beer barrel due to a pressure attributed to the leakage of internal gas.

CONSTITUTION: A fixed lid 27 is mounted airtightly sealed to a mouth section 10 of a draft beer barrel 9 by use of a rubber 32 to detect a gas leakage due to deficiency of a fitting 8 provided on the mouth section 10 thereof. The lid 27 has a communicating hole 33 on the bottom thereof and a pipe 36 connected to the hole 33 has branch tubes connected to an air-open solenoid valve 34 and a liquid feed solenoid valve 35, and connected to a U-shaped tube 37 ahead thereof. An electrode 38 is mounted to the U-shaped tube 37. A beer barrel 9 being carried on a conveyor is halted at the home position and a liquid from the solenoid valve 35 is introduced into the pipe 36 and the U-shaped tube to wash. Then, the valve 34 is opened to seal a liquid only into the U-shaped tube 37 and the lid 27 is mounted tightly on the mouth section of the barrel 9. When any gas leaks at the fitting 8, the liquid in the U-shaped tube 37 is pushed up, a condition which is detected with an ammeter connected to the electrode 38.