To accurately measure the amount of heaving without influenced by the inclination of a floating body or the shaking of a gimbals.

An acceleration sensor 4 detects a Z-axis component of acceleration in a rectangular coordinate system. A gyro sensor 5 detects an angle of X-axis inclination, and a gyro sensor 6 detects an angle of Y-axis inclination. The acceleration sensor 4, the gyro sensor 5 and the gyro sensor 6 are secured on a supporting plate 3 attached to a gimbals mechanism 2, which is constructed rockably back and forth or from side to side. An acceleration detected by the acceleration sensor 4 is corrected based on both the angle of X-axis inclination detected by the gyro sensor 5 and that of Y-axis inclination detected by the gyro sensor 6, and then integrated twice in an integral filter to measure the amount of displacement (or heaving).