PURPOSE: To enable a bow part to be easily manufactured under good workability, by preparing blocks separately as the left and right block facing upward its part, being used for an inboard side, and inverting one block to be piled on and welded to the other block.

CONSTITUTION: A shell 3 and each aggregate 4W7 are jointed to each other preparing left and right bulbous bows in the bow part separately as a block 8a, 8b facing upward its part being used for an inboard side. After completion of each block, one block 8b is inverted to be piled on the other block 8a, and their connection part is welded. By this method, the bulbous bow of complicated shape as a whole in the bow part can be easily prepared by enabling the shell 3 and each aggregate 4W7 to be jointed and welded under good workability in which the jointing and welding work is performed facing downward.