PURPOSE: To facilitate centering of a conduit by a method wherein a stern having an upper half part of the conduit is formed by applying FRP to a stern mold, and a lower half part of FRP conduit formed separately is joined with the stern.

CONSTITUTION: A stern 6 consisting of a stern bottom 1, an upper half part 3 of conduit for hydrojet propeller, riser walls 41, 42 along generating line O1'-O2' is formed by applying FRP to a stern mold. The backs of the riser walls are cavities 51, 52. On the other hand, an FRP lump 10 provided with a groove 8 and a wall 9 constituting a lower inside wall 72 correspondingly to a generating line O3-O4 of a conduit upper inside wall 71 is prepared. The lump 10 is inserted in the cavities 51, 52 for assembly, and both two are joined by FRP.