A ship propulsion system comprising: a plurality of main generators (10A-C) configured to supply electric power to facilities in a ship; an electric power distribution unit configured to distribute the electric power from the main generators (10A-C); a main engine configured to rotate a main propeller by allowing a first rotary shaft to be driven to rotate; a first electric motor configured to compensate for a shortfall in the rotation of the main propeller by the main engine by allowing the first rotary shaft to be driven to rotate by the electric power distributed by the electric power distribution unit, the first electric motor including a generator configured to generate electric power from the rotation of the first rotary shaft resulting from output of the main engine and to supply the electric power to the electric power distribution unit; a second electric motor allowing a second rotary shaft to be driven to rotate by the electric power distributed by the electric power distribution unit; a stern-side propeller placed on a stern side of the main propeller and arranged to rotate with the second rotary shaft; and a control device configured to control the first electric motor to switch operations between allowing the first rotary shaft to be driven to rotate by the first electric motor in response to thrust required for the ship and supplying electric power to the electric power distribution unit by generating the electric power from the rotation of the main engine, and the control device configured to control the electric power distribution unit to distribute the electric power from the main generators (10A-C) to facilities in the ship, to the first electric motor, and to the second electric motor when the first electric motor drives to rotate the first rotary shaft, and to distribute the electric power from the main generators (10A-C) and the generated electric power from the generator of the first electric motor to facilities in the ship and the second electric motor when the first electric motor generates the electric power. a plurality of main generators (10A-C) configured to supply electric power to facilities in a ship; an electric power distribution unit configured to distribute the electric power from the main generators (10A-C); a main engine configured to rotate a main propeller by allowing a first rotary shaft to be driven to rotate; a first electric motor configured to compensate for a shortfall in the rotation of the main propeller by the main engine by allowing the first rotary shaft to be driven to rotate by the electric power distributed by the electric power distribution unit, the first electric motor including a generator configured to generate electric power from the rotation of the first rotary shaft resulting from output of the main engine and to supply the electric power to the electric power distribution unit; a second electric motor allowing a second rotary shaft to be driven to rotate by the electric power distributed by the electric power distribution unit; a stern-side propeller placed on a stern side of the main propeller and arranged to rotate with the second rotary shaft; and a control device configured to control the first electric motor to switch operations between allowing the first rotary shaft to be driven to rotate by the first electric motor in response to thrust required for the ship and supplying electric power to the electric power distribution unit by generating the electric power from the rotation of the main engine, and the control device configured to control the electric power distribution unit to distribute the electric power from the main generators (10A-C) to facilities in the ship, to the first electric motor, and to the second electric motor when the first electric motor drives to rotate the first rotary shaft, and to distribute the electric power from the main generators (10A-C) and the generated electric power from the generator of the first electric motor to facilities in the ship and the second electric motor when the first electric motor generates the electric power.