PURPOSE: To observe the advancing direction of waves, by mounting inclinometers on two horizontal axes of a disk-shaped buoy in order to measure pitching and rolling, and measuring the direction of the buoy by means of a compass attached thereto.

CONSTITUTION: A disk-shaped buoy 1 is provided with a magnetic compass 2 to measure the direction of the buoy 1 with respect to the magnetic north, an inclinometer 3 mounted on the buoy horizontal reference axis (Y-axis) and an inclinometer 4 mounted on the horizontal axis (X-axis) perpendicular to the Y-axis inclinometer 3. The angular difference between the horizontal reference axis of the buoy and the direction of the magnetic north is detected by the magnetic compass 2, and the measured value is outputted as the value with respect to the magnetic north. By such constitution, if the buoy 1 rotates during measurement, no measuring error appears. Calculation is performed by an angle calculating circuit 10, a vector calculating circuit 11, a subtracting circuit 9 and the like.