PURPOSE: To enable accurate measurement of the quantity of liquid by detecting the position of a specified place of the diaphragm or the length of the diaphragm contacting with the side wall of the main body of the tank and thereby measuring the quantity of the stored liquid.

CONSTITUTION: A rope 5 is fitted to the central point A of the diaphragm 2 of the tank 1 with the diaphragm, drawn out of the main body 1 of the tank and given a tensile force by a spring 6 so as not to slacken. The length of movement of the rope is measured continuously, whereby the quantity of the storage of oil 3 which coexist with water 4 with the diaphragm 2 between them is measured. Or, numerous supersonic sensors 10 are arranged on the side wall of the main body 1 of the tank, the length L of the diaphragm contacting with the side wall is detected by a relay 11 and a supersonic detecting device 12, and the quantity of oil 3 is indicated in an indicator 13. By this, the quantity of stored oil can be measured accurately.