To solve problems of labor and labor hours, and degree of risk in a welding part or a R part in building a ship, or a pollution problem of an abandoned FRP ship.

A recyclable and weldable material without causing pollution is used. The material necessary depending on the size of the ship is divided in a strip plate shape, shifted, and superimposed, and its end is welded. A reinforcing plate is attached to the backside end, and mutual intensity is also increased by welding the present plate and the next plate. It is highly resistant to an impact from the inside and the outside. The ship can be built with a plate thickness of 1/2 to 2/3 compared with that of a conventional one by this construction method using rivets and spot welding together. Thus, wasting labor, injury, or danger caused by handling of a conventional large and heavy material can be reduced drastically. The shifted and superimposed material is connected by the reinforcing plate and fastened by a square rivet.