To enhance stability and astern power by abolishing a reservoir.

This propulsion device has a normal/reverse rotation converter 16 placed between the driveshaft 14 of an impeller 7 and the output shaft 15 of a driving engine 3 and provides large astern power by submersing the rear end of a deflector 9 provided behind a delivery casing 8, then pressurizing water sucked from the deflector 9, and ejecting the water directly from the suction opening 5 of a suction casing 4 in a direction diagonally forward of the bottom of the vessel. Steering rudders are placed on both sides of the deflector 9 to enhance a course keeping property and steerability astern in the same manner as when the vessel goes ahead and at the same time to protect the deflector 9 from floating objects. A screw blade having a series of continuous torsional blade faces is provided in a coned flow passage to pressurize and increase the speed of water flow in a spiral centrifugal direction so as to enhance suction performance by means of high thrust in the flow passage due to the speed of ejection and by means of inducer action at the suction part of the screw blade, so as to prevent cavitations.