To provide a propelling machine for a ship to smoothly reduce the number of revolutions of an engine when a throttle valve is rapidly closed from a wide opening state.

This propelling machine 1 for a ship is formed such that the crank shaft 3 of a fuel injection type four-cycle engine 2 rotates a propeller 4 through a shift mechanism 11 and a throttle valve 54 is mounted on a suction pipe 52 through which air is guided in the cylinder 7 of the engine. A bypass flow passage 55 bypassing the throttle valve is formed, an ISC valve 81 is situated in the bypass flow passage. When the throttle valve is rapidly closed from a state that the valve is widely opened, the ISC valve is closed from an opening state. The closing speed of the ISC valve when a shift mechanism is switched to a neutral is decreased to a value lower than that when the shift mechanism is shifted to forward.