A control device of a marine propulsion device, which has an engine and a propeller and is mounted on a hull. The control device includes a processor, and a non-transitory storage medium having program instructions stored thereon. The execution of the program instructions by the processor causes the control device to acquire a rotation speed of the engine or a rotation speed of the propeller, acquire a torque of the engine, acquire a speed of the hull, and determine whether a ventilation of the engine has occurred based on at least two among the acquired rotation speed, the acquired engine torque, and the acquired hull speed.