To efficiently measure a horizontal distribution of tidal current over wide range in a short time.

Flow velocity of the tidal current at a target point TP is measured by each of ships S1-S3, and measured data and position data of the ships are transmitted from other ships S2, S3 to the ship S1 forming a base station. In the ship S1, flow velocity vector V of the tidal current at the target point TP is obtained based on these data and data measured by the ship S1 itself, The horizontal distribution of the tidal current can be obtained by performing this operation at each target point.