To restrain the lowering of propulsion efficiency of a rear-side propeller due to upward swirl flow of water formed by a front-side propeller around in the neighborhood of the center line, in a twin-screw vessel having overlapping propellers that rotate outward.

A fin 8 is disposed, at a position located nearer to the bow side than the front-side propeller 5-2, in between the front-side propeller and rear-side propeller 5-1 in the right-to-left or vice versa direction. The fin is designed to have an elevation angle that inclines down toward the stern side. This fin adds a downward component to water stream so that the propulsion efficiency of the rear-side propeller is restrained from lowering.