PURPOSE: To measure the position of the interface between oil and water and to make it possible to detect the oil quantity, by detecting the pressure difference between the inside and outside of a tank at the same horizontal level.

CONSTITUTION: In an oil and water displacement type sea bottom tank, crude oil is received and an oil water interface 16 is formed. An oil layer and a displaced water layer are balanced. Under this state, a pressure detecting end 21 is provided at the outside of the tank and at the same horizontal position as the position of a pressure detecting end 20, which is attached to the inner wall at the top part of the tank. The distance between said position and the sea surface 2 is expressed by h1. The distance between the position and the oil water interface 16 is expressed by h2. The specific gravity of the sea water is 1.03 and that of the crude oil in the tank is 0.85. Then the pressure balance becomes as follows: the water pressure applied upward to the oil water interface 16 is (h2+h2)×1.03; and the load of the oil applied to the oil water interface 16 becomes h2×0.85. Then the pressure applied to the detector 20 at the inner wall of the top part of the tank is obtained. Since the pressure of the sea water applied to the outer wall of the top part of the tank is h1×1.03, the pressure difference between the inside and outside of the outer wall of the tank at the same level is 0.15×h2. The pressure difference is detected and divided by the difference in specific gravities. Thus the depth of the oil layer can be automatically measured.