PURPOSE: To reduce the revolving component of the flow of water injected through an injection nozzle and to improve propulsive efficiency by a method wherein a guide vane for straightening the revolving flow into a parallel flow, a groove, and other flow straightening means are situated inside the outlet part of the injection nozzle mounted to a stern.

CONSTITUTION: A water jet pusher is provided with a water guide duct 34 to guide water taken in through an intake 32, formed in the bottom part of a boat 31, to the interior of a pump casing 33, a pump impeller 35 disposed in the pump casing 33, and a diffuser 36 connected to the secondary side of the pump casing 33. The diffuser 36 is provided with a guide vane 38 to straighten a pressurized water flow and guide the water flow to an injection nozzle 37 and a needle 39. In this case, a detector 41 to detect the revolving component of a water flow is mounted on the downstream side of the guide vane 38, and an auxiliary guide vane 43 serving as a flow straightening means, the opening of which is regulated by a drive device 46 controlled according to an output from the detector 41, is mounted on the downstream side of the detector.