PURPOSE: To realize the space economy by molding hull and deck as an unit with a space below the deck as a store room, and installing an opening connecting inside and outside of the store room for ventilating thus simplifying the installation of ventilating equipment.

CONSTITUTION: Hull A and deck d is molded as a monolithic unit with FRP. Deck (d) is provided with a step 2 thereby forming a store room (e), while the standing face 2a of the step 2 is provided with a hollow projection 6 molded simultaneously with deck (d) and projecting externally. Plate form material 9 is connected to the inside of hollow projection 6 covering the major part from the lower end to the upper part. Ventilation of store room is performed through the opening 7 provided on the projection 6 and the upper portion of the hollow projection 6.