An observer measures a flow velocity of sea surface with beam, and a coastal wave height. A predictor predicts a next time state vector, from a state vector including flow velocity of each range cell of beam, a wave height difference between range cells, and a coastal wave height. First calculator calculates a prediction error covariance matrix from a smoothing error covariance matrix. Second calculator calculates a gain matrix using results obtained by the observer and the first calculator. Third calculator calculates a smoothing error covariance matrix using results by the observer, the second calculator, and the first calculator. The state vector is smoothed for each wave height difference, from results by the observer, the predictor, and the second calculator. The wave height of each range cell is calculated by adding the wave height and the wave height difference in toward-offshore direction, using the wave height difference smoothing.