PURPOSE: To eliminate wasteful discharge of water stored in a water tank and to reduce working hours for inspection in the case operation inspection of a water level detection device provided to a water receiving tank is made.

CONSTITUTION: A testing cylinder 21 surrounding inspection bars 12-15 for detecting specific water level is provided. The testing cylinder 21 is capable of connecting the inside thereof to the outside with connection holes 22. In the case of connection, the water level in the testing cylinder 21 is equal to the water level in the water tank 2, and a water receiving tank 1 is normally used in such a state. In the case operation inspection of a water level detection device is made, the connection holes 22 are closed, and the inside and outside of the testing cylinder 21 are cut off. At that time, the water level in the testing cylinder can be only varied irrespective of the water level in the water tank 2.