To provide a thruster with a duct capable of securing stable towing force during a low speed work and improving propulsive efficiency by preventing peeling vortexes on an external surface of the duct during a high speed navigation.

In the thruster, a swelling part 6 swelling outward from a standard wing shape in a circular-arc shaped cross section is provided on an outer periphery at a front end part of the duct 5 so that a cross-sectional shape of the duct of a wing shape cross section provided around a propeller 4 may suppress a pressure change on the external surface at the front end part of the duct during the high speed navigation. The duct 5 includes an opening angle α spreading in the front end direction so that the prescribed towing force may be exerted during the low speed work.