PURPOSE: To accurately measure the relative level between a plurality of blocks and to easily unify the measuring level of a measuring area.

CONSTITUTION: Displacement manometers 2 detecting predetermined water levels are respectively arranged in a plurality of blocks A, B formed by dividing a measuring area and reference manometers 1A, 1B continuously detecting water levels are respectively provided to the respective blocks to be allowed to communicate with the displacement manometers 2 through communication pipes 4. A displacement manometer 3A for detecting a relative level is arranged to the pillar 5 between the adjacent blocks A, B and the displacement manometer 3B communicating with the displacement manometers 2 of the blocks B to detect the relative water level is arranged to the pillar 5.