Ship speed gauge and method for obtaining ship speed

Provided is a ship speed gauge capable of measuring log speed over water with good accuracy at each water depth up to depths at which log speed over water sufficiently develops. According to a representative embodiment, fluid speeds in a fluid on which a ship is floating are measured up to a predetermined measurement range at multiple different water depths, an already retained log speed over water at each water depth logical curve is fitted to a distribution of the fluid speeds at each water depth and a log speed over water at any water depth in a region deeper than the measurement range is calculated on the basis of the fitted logical curve.