An outboard motor adapted to be mounted on a stern of a boat and having a propeller to propel the boat, a power source which produces rotational output, a drive unit including a vertical shaft which transmits the rotational output of the power source to the propeller through a gear mechanism, and an interlock unit which detachably interlocks the power source to the drive unit. The changeable power source includes one of an internal combustion engine, an electric motor and a combination of the engine and the motor. The interlock unit interlocks one of the engine, the motor and the combination of the engine and the motor to the drive unit as the power source for propelling the boat, thereby enabling to freely change its power source as desired in accordance with a user's current needs and purposes.