PURPOSE: To provide a lightweight water jet propulsion device with no cavitation so as to enhance the efficiency and the quietness in which a positive displacement pump part is formed in a scroll type composed of a stationary spiral element having a spiral groove formed in a thick plate, and a movable spiral element having a spiral protrusion formed on an end plate.

CONSTITUTION: A spiral protrusion 1a formed on a movable spiral element 1 and a spiral groove 2a formed in a stationary spiral element 2 are combined with each other so as to form a pump part, and the stationary spiral element 2 is fixed to a hull. When the movable spiral element 1 is turned with a substantially constant eccentricity and without self-rotation by means of a drive shaft 8, water is sucked into the pump part through a suction port which is communicated with the outer terminal end part of the spiral groove 2a and opened at the outer wall, and is discharged under a high pressure through a discharge port 2d which is formed in a bulb-like protrusion located a substantially center part of the groove and opened to the latter. At this time, the discharged water passes through a rudder 5 provide in a nozzle in the stationary spiral element 2, thereby it is possible to generate a propulsion for a marine vessel.