To provide a propeller-water-flow improving device capable of enhancing a propulsion efficiency of a ship by efficiently using the eddy water flow in propeller rear flow of a ship.

For efficiently using the propeller-water-flow on which no attention has been focused, an outer periphery of a propeller 3 is surrounded by a cylinder 1 from the front part to the rear part of the propeller 3. In addition, a water flow outlet 2 is provided for the cylinder 1 so that the water flow can flow out of the cylinder 1 at higher water flow speed than water flow speed in the cylinder 1 behind the propeller 3 at the time of flowing the water flow out of the cylinder 1.