PURPOSE: To permit easy and accurate decision of the boundary surface between two kinds of liquids having different specific weights by a system wherein specific weight is determined from a difference in liquid pressure at vertically different two points of a piping for expelling the lighter liquid above the ground in an underground tank used for storing those two liquids.

CONSTITUTION: A difference in liquid pressure at different two points of a piping for expelling a lighter liquid above the ground from an underground tank used for storing two kinds of liquids having different specific weights is measured and then specific weight of a liquid within the piping is computed. Thus obtained value is compared with a predetermined specific weight value so as to detect the boundary surface between the two liquids. For example, low and high pressure diaphragm LD, HD are mounted at vertically different two position A and B of piping PP for expelling the lighter liquid above the ground, respectively. A difference in liquid pressure between those two positions is detected by a differential pressure transmitter TP including a diaphragm tank via a capillary CA and specific weight is computed by an arithmetic unit AL. A change in the specific weight due to that the expelled liquid becomes a mixed liquid of oil and water is measured so as to detect that the expelled liquid within the tank reaches the boundary surface between oil and water.