A ship's speed meter for measuring a speed relative to the water of a ship 10, the ship's speed meter including a wave transmitter 1 for emitting a sound wave toward a sea bottom 20, a wave receiver 2 for detecting a plurality of reflected waves, which are reflected waves of the sound wave having been emitted from the wave transmitter 1, reflected by a plurality of reflecting objects 30 positioned at different water depths, and an arithmetic processing unit 4 for calculating a ship's speed relative to the water of the ship 10 based on a frequency difference of the sound wave and the reflected wave. The arithmetic processing unit 4 obtains a change rate of a current velocity in a water depth direction by obtaining current velocities at a plurality of different water depths based on a frequency difference between the sound wave and the plurality of reflected waves, and calculates a current velocity at a water depth at which the change rate is smaller than or equal to a threshold value as the ship's speed relative to the water of the ship 10.