To suddenly accelerate a water jet propulsive ship in a resting state or in a cruising state at very low speed.

A ship hull 1 is equipped with a water jet propulsion device 2 having a water jet conduit line 5 running from a water intake 3 at ship bottom to a blow-out opening 4 at its stern, and a propulsion pump 6 so as to configure the water jet propulsive ship. A water tank 18 is installed on the ship hull 1 and a water supplying conduit line 19 with a booster pump 20 is located close to an upstream side of the propulsion pump 6 in the water jet conduit line 5 so as to configure a water supplying device I. At the time of accelerating the water jet propulsive ship in a resting state or in a cruising state at very low speed, water in the water tank 18 is pressurized by the booster pump 20 and fed to the water jet conduit line 5 so that enough water can be sucked in when a rotary drive speed of the propulsion pump 6 is raised, and a lot of water can be jetted out from the blow-out opening 4 of the water jet propulsion device 2.