To correctly reduce or prevent the gear noise generated in a propulsion system caused by the fluctuation in rotation of an engine.

This marine propulsion system in which the power is transmitted to an input pinion shaft 7 of the propulsion system 5 from the engine 2 via a connection shaft 10, and an input pinion 16 of the input pinion shaft 7 is engaged with forward and backward gears 18 and 19 in the propulsion system 5 comprises a neutral condition detection mechanism 35 for detecting the neutral condition of the propulsion system 5 and a braking actuator 31 for braking the input pinion 16 or component on the drive side only during the neutral condition. The neutral condition detection mechanism 35 comprises a proximity switch 36, a detection lever 38, etc., and when the neutral condition is detected, the braking actuator 31 is operated, the component on the drive side such as a flange 33 of a joint shaft 11 is braked, and generation of the gear noise is reduced or prevented.