PURPOSE: To detect a leakage accurately independently of charge and discharge flow rates to and from a liquid preservation tank and display the quantity of the leakage, by detecting the difference between charge and discharge flow rates to and from the tank and the variation of the liquid level in a unit time in respect to detection for leakage of the liquid preservation tank.

CONSTITUTION: The flow-in rate to a tank 1 and the flow-out rate from the tank 1 are detected in unit time flow rate integrating devices 10 and 11, and their difference is operated in a flow rate difference operator 14. A liquid level is detected in a level transmitter 17, and the variation of the net capacity is operated in a unit time variation measuring device 19 and a variation operation 21. The difference between outputs of two operators 14 and 21 is operated in a leakage quantity operator 16. The quantity of leakage is displayed on an indicator 26, and a preliminarily set value and the output of the leakage quantity operator 16 are compared with each other to operate an alarm device 28 as required. Though only the lowering of the liquid level is monitored in the conventional method, an abnormality is detected quickly and accurately in this method.