PURPOSE: To discriminate the presence or absence of leakage of a container in a quick and accurate way, by inspecting the state of reduced pressure in the container within a prescribed test time after setting the air pressure in the container at the test pressure.

CONSTITUTION: The air discharged out of a pump 1 is branched to a working machine system circuit A and a test system circuit B. In the circuit B, a valve 11 is opened in a state under which the air reduced down to the set pressure by a valve 12 is supplied to a pipeline 13. As a result, the air having the pressure higher than the test pressure P is led into a container C through a pipeline 15. When the inside of the container C reaches a prescribed level, the valve 11 is closed by a switch 22. Then a valve 16 is opened to slowly increase the pressure in the container C. After this, the valve 16 is closed when the pressure at the inside of the container C reaches the set level of a switch 23 which is higher than the pressure P. Then a valve 18 is opened to extract the air until the pressure inside the container C reaches the set level of a switch 24. Thus the valve 18 is closed when the pressure at the inside of the container C reaches the test pressure. Then a timer works to start a test of leakage.