To simplify mechanism and to reduce cost by providing a water jet nozzle part provided at the stern, with a nozzle plate adjustable by moving in the cross direction of the nozzle, and providing the nozzle plate with a plurality of injection nozzles of different bores opened in parallel.

A jet propulsion device forms a jet water passage by a jet water suction casing 2a, a pump casing 2b, a discharge casing 2c and a injection nozzle casing 2d. A nozzle plate 8 has large, medium and small injection nozzle 8a, 8b, 8c opened in parallel in one nozzle plate 8a, and fitted to be slidable at a neck part of the injection nozzle casing 2d, and the injection nozzle 8a, 8b, 8c are selectively used. The injection bore of the injection nozzle casing 2d can therefore be easily changed so as to be able to sail injecting optimum jet by selecting the injection nozzle according to the grade of live load.