In an outboard motor adapted to be mounted on a stem of a boat and having a propeller to propel the boat, there are provided 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 one of an internal combustion engine, an electric motor and a combination of the engine and the motor to the drive unit as the power source, thereby enabling to freely change its power as desired in accordance with purposes.