A marine propulsion device, comprising: a power transmission clutch provided on an input shaft to transmit the driving force from a main engine; a horizontal input/output shaft, one end side of which is connected to the input shaft through the power transmission clutch; a vertical shaft, one end side of which is connected to a substantially central part of the input/output shaft through a first turning mechanism; a horizontal propeller shaft, one end side of which is connected to the other end side of the vertical shaft through a second turning mechanism; and a propeller fitted to the other end side of the propeller shaft, in which the propeller is driven by the main engine to propel a ship, and the propeller shaft and the propeller are revolved around the vertical shaft to set a propulsion direction of the ship, wherein the marine propulsion device further comprises: a motor generator connected directly to the other end side of the input/output shaft; a two-way inverter connected to a power supply to control the motor generator; characterized by a controller (40; 40'; 40"), which propels the ship in a propulsion mode selected from motor propulsion in which with the power transmission clutch put in a disengaged state, the motor generator is driven as a motor and hybrid propulsion in which with the power transmission clutch put in the connecting state, the motor generator is used as a motor or a generator while the drive is performed by the main engine, wherein the controller: performs an operation on receiving input of a handle command signal and load information on the main engine, the handle command signal being output according to the number of revolutions of the propeller designated by a ship operator handling handle, outputs a mode select signal for setting a control mode of the motor generator to a revolution number control mode or a torque control mode and an inverter command signal for driving the motor generator in the selected control mode to the two-way inverter according to the arithmetic result, further outputs a clutch control signal for putting the power transmission clutch in the connecting state or the disengaged state to the power transmission clutch, and further outputs a governor command signal for giving a command of revolution number of the main engine to the main engine, to thereby control the motor generator in the revolution number control mode in the motor propulsion in a motor propulsion region in which the number of revolutions of the propeller is equal to or less than a switching predetermined number of revolutions, and control the main engine and control the motor generator in the torque control mode in the hybrid propulsion in a hybrid region in which the number of revolutions of the propeller is equal to or more than a switching predetermined number of revolutions. performs an operation on receiving input of a handle command signal and load information on the main engine, the handle command signal being output according to the number of revolutions of the propeller designated by a ship operator handling handle, outputs a mode select signal for setting a control mode of the motor generator to a revolution number control mode or a torque control mode and an inverter command signal for driving the motor generator in the selected control mode to the two-way inverter according to the arithmetic result, further outputs a clutch control signal for putting the power transmission clutch in the connecting state or the disengaged state to the power transmission clutch, and further outputs a governor command signal for giving a command of revolution number of the main engine to the main engine, to thereby control the motor generator in the revolution number control mode in the motor propulsion in a motor propulsion region in which the number of revolutions of the propeller is equal to or less than a switching predetermined number of revolutions, and control the main engine and control the motor generator in the torque control mode in the hybrid propulsion in a hybrid region in which the number of revolutions of the propeller is equal to or more than a switching predetermined number of revolutions.