PURPOSE: To prevent vibration of a ship hull and the generation of noise due to the vibration by supporting both the casing of a water jet unit and a power transmission system with a vibration insulating member therebetween in a state to separate the two members away from the ship hull, and resiliently intercoupling a crank shaft and the power transmission shaft system.

CONSTITUTION: A water jet propulsion device is formed such that both an engine and the impeller of a water jet unit 11 through a power transmission shaft system 13 extending through a partition wall 3. In this case, in the power transmission shaft system 13, vibration of an input shaft 36 is absorbed by a vibration insulating member 45 located between an outer case 39 of a front bearing unit 38 and an inner case 44. A flange part 48a of a coupling shaft 48 secured to the front end of the input shaft 36 is coupled to a flange part 35b of an output shaft 35a, spline-coupled to the end part of a crank shaft, through a coupling 49 made of rubber. Further, the water jet unit 11 is securely fastened against a ship hull 2 through a vibration insulating member 61 previously incorporated in a leg part 60 for securing.