PURPOSE: To obtain a good propelling efficiency even when a boat cruises at a speed other than a high speed by composing a nozzle provided at the stern with a main nozzle of a constant jet orifice, and an outer nozzle whose jet orifice is expanded or contracted, provided at the outer side of the main nozzle.

CONSTITUTION: A water jet propeller gets a propulsion by leading the water taken in from a water input port 32 provided at the bottom of a vessel 31 to a diffuser 36 through a water leading duct 34 by the rotation of a pump runner 35, and after rectifying with a guide vane 38, spraying it from a jet nozzle. In this case, the jet nozzle is composed of a main nozzle 37 of a contracted diameter type with the constant jet orifice, and an outer nozzle 41 provided at the outer side of the jet orifice of the main nozzle 37. And the outer nozzle 41 is composed of plural nozzle pieces 42 which gear with a tapered surface 37a formed at the outer end surface of the jet orifice of the main nozzle 37 slidable and movable in the diameter direction, and divided in the circumferential direction, being expanded or contracted freely by a hydraulic driving means 43.