A marine vessel propulsion device attachable to a marine vessel and comprising: a duct; a propeller (6, 206, 406, 606, 706) that is rotatable with respect to the duct around a propeller axis, the propeller including a plurality of blades and a rim (16, 447, 449) that surrounds the plurality of blades, the propeller being surrounded by the duct; and an electric motor (7, 307, 407) that includes a stator (24, 454, 456) and a rotor (25, 455, 457), and that rotates the propeller (6, 206, 406, 606, 706) by rotating the rim (16, 447, 449) with respect to the duct, a bracket that is attachable to the marine vessel, wherein the duct is rotatable around a vertical or substantially vertical steering axis with respect to the bracket, the propeller axis extending in a direction perpendicular or substantially perpendicular to the steering axis, and a motor electronic control unit is mounted in the marine vessel propulsion device, said motor electronic control unit is electrically connected to the electric motor (7, 307, 407) and adapted to control said electric motor (7, 307, 407); and an output adjusting device is adapted to perform an output adjustment of the marine vessel propulsion device, wherein the output adjusting device is electrically connected to the motor electronic control unit, wherein the electric motor (7, 407) includes the stator (24, 454, 456) defined by at least one portion of the duct and the rotor (25, 455, 457) defined by at least one portion of the rim (16, 447, 449), and based on an output command that has been input from an output adjusting device, the motor electronic control unit is configured to control a power supply to the stator, characterized in that the marine vessel propulsion device comprises a motor rotation angle detector detecting the rotation angle of the rotor with respect to the stator, so that based on an output generated by the motor rotation angle detector, the motor electronic control unit is configured to control the power supply to the stator 24, and hence configured to control a rotation direction and the rotation speed of the rotor. a duct; a propeller (6, 206, 406, 606, 706) that is rotatable with respect to the duct around a propeller axis, the propeller including a plurality of blades and a rim (16, 447, 449) that surrounds the plurality of blades, the propeller being surrounded by the duct; and an electric motor (7, 307, 407) that includes a stator (24, 454, 456) and a rotor (25, 455, 457), and that rotates the propeller (6, 206, 406, 606, 706) by rotating the rim (16, 447, 449) with respect to the duct, a bracket that is attachable to the marine vessel, wherein the duct is rotatable around a vertical or substantially vertical steering axis with respect to the bracket, the propeller axis extending in a direction perpendicular or substantially perpendicular to the steering axis, and a motor electronic control unit is mounted in the marine vessel propulsion device, said motor electronic control unit is electrically connected to the electric motor (7, 307, 407) and adapted to control said electric motor (7, 307, 407); and an output adjusting device is adapted to perform an output adjustment of the marine vessel propulsion device, wherein the output adjusting device is electrically connected to the motor electronic control unit, wherein the electric motor (7, 407) includes the stator (24, 454, 456) defined by at least one portion of the duct and the rotor (25, 455, 457) defined by at least one portion of the rim (16, 447, 449), and based on an output command that has been input from an output adjusting device, the motor electronic control unit is configured to control a power supply to the stator, characterized in that the marine vessel propulsion device comprises a motor rotation angle detector detecting the rotation angle of the rotor with respect to the stator, so that based on an output generated by the motor rotation angle detector, the motor electronic control unit is configured to control the power supply to the stator 24, and hence configured to control a rotation direction and the rotation speed of the rotor.