In a propulsion device for a marine motor, a shift rod (19) extends vertically in a gear case (1a) provided in a lower part of the marine motor, and is provided with a lower end (19a) cooperating with a mechanism for mechanically actuating a clutch device (18) for shifting a power transmission mechanism of the propulsion device. The lower end of the shift rod is additionally provided with a valve (37, 38) that controls feeding of hydraulic oil to an actuator (36) for assisting an effort required to turn the shift rod and actuate the clutch device. Thereby, the manual torque applied to the shift rod to turn the same to a forward and reverse position is assisted by the hydraulic actuator with a minimum modification to a purely manual arrangement for shifting the power transmission mechanism.