To prevent the occurrence of a part, having a high curvature, to the mounting part of a stern base plate, through simple structure.

In a water jet propulsion boat made of FRP wherein a water jet pusher is installed at a stern bottom, a stern base plate 15 is recessed in an up state and a space in which the jet pusher is installed is formed in a recessed part 160. The stern base plate 5 is attached to the lower end part of the recessed part 160 and a bottom is formed of a stern base plate 5. The base plates 15 on both sides of the recessed part 160 and the rise walls 16 on both sides of the recessed part are interconnected through a flat inclination plate 18. Formation is such that an angle between the inclination plate 18 and the rise wall 16 and an angle between the inclination plate 18 and the base plate 15 are set to an obtuse angle, and the flange surfaces 520 of the two side parts of the stern base plate 5 are brought into contact with the two inclination plates 18 to fix the stern base plate 5.