PURPOSE: To accurately measure the relative level between a plurality of blocks and to reduce the measuring error included in a measuring level to be unified.

CONSTITUTION: Displacement manometers 2 for detecting a predetermined water level are arranged at the respective measuring points in blocks A, B and reference manometers 1A, 1B for continuously detecting a water level are respectively provided to the blocks A, B to be connected to the displacement manometers 2 through communication pipes 4. The displacement manometer 3A of the block A for detecting a relative level and the displacement manometer C of the block B are arranged to the pillar 5 of the block A and the displacement manometer 3B of the block A and the displacement manometer 3D of the block B are arranged to the pillar 6 of the block B. The differences between the relative levels of the displacement manometers 3A,3C and the displacement manometers 3B,3D are averaged by a controller 7 and the levels of respective measuring points detected by the displacement manometers 2 are averaged to unify measuring levels on the basis of the relative level differences.