PURPOSE: To simplify the structure of a hull, firmly assemble the hull and facilitate work.

CONSTITUTION: A slip body 2 is formed of a hull 3, deck 4 covering over the hull 3, and a floor 5 which is formed integrally on the inside of the deck 4, and the upper part 3a of the hull 3 and the lower part 4a of the deck 4 are superposed and joined, and an opened port part 6 is formed on the side wall 5a of the floor 5 corresponding to the superposed part A, and a reinforcing member 7 which connects the upper part 3a of the hull 3 of the superposed part A and the lower part 4a of the deck 4, from the opened port part 6, and reinforces them is installed from the inside of the slip body 2.