To provide a power transmission structure of a main engine for a ship suitable for high speed operation by reducing the weight.

The main engine 10 and a decelerator 30 are connected with a first power transmission shaft 20. As the distance between the main engine 10 and the decelerator 30 is large, a bearing 60 is mounted in the middle of the first power transmission shaft 20 to reduce the vibration caused by the deflection of the shaft. The decelerator 30 and an impeller 40 are connected with a second power transmission shaft 50 whose diameter is larger than that of the first power transmission shaft 20. The rotational frequency of the main engine 10 is reduced by the decelerator 30 and the torque is increased. The power of the main engine 10 is transmitted to the impeller 40 to drive the water jet.