A flow control mechanism is provided for controlling a stern flow field disturbed by cooling water discharged from the cooling water outlet of a ship. The flow control mechanism is located within a distance, which corresponds to three times the diameter of the cooling water outlet, from the cooling water outlet, is installed at an angle from 16° to 30° with respect to the horizontal plane of a hull, and has a vertical height, which is within a range from 0.2 times to 0.6 times the diameter of the cooling water outlet in a vertical direction of the hull, and a horizontal width, which is within a range from 0.5 times to 1.3 times the diameter of the cooling water outlet in a horizontal direction of the hull. Thereby, the flow control mechanism restores the distribution of wake flow that encounters a propeller so that the distribution of the wake flow is symmetrical around the central axis of the hull, and reduces the generation of a cavity. Thus, the vibration performance of the ship is improved, and the propulsive efficiency of the ship is also improved.