PURPOSE: To continuously detect the flow rate measuring water level of a liquid tube column, by providing an pneumatic pressure detector fro detecting the change of air pressure in a tube-like probe as the change of the water level of the liquid tube column.

CONSTITUTION: A tube-like probe 1 is directly connected to a maximum water level detector 4 and coupled to a minimum water level detector 2 through a branched conduit 13. With change of the water level of a water tube 12 fluctuation is produced at the water level L3 of the probe 1 in response thereto, and air entering the probe 1 is affected by change of pressure proportional to the fluctuation. Next, when the water level of the tube 12 increases, air compressed through a conduit 14 pushes up to the point of a top plate 8 which can be slid in the inside of a sliding cylinder 9. A maximum switch 6 for controlling a pump 8 through a running relay 7 is operated, and the position of the maximum water level 12 can be set. In addition, when the water level of the tube 12 is lowered down to the minimum level L1, the detectors 2, 4 are lowered and when the detector 2 opens a switch 3, a pump P receives a stop instruction through the relay 7.