PURPOSE: To ensure sealing property sufficiently so as to prevent leakage of water into a space between outer and inner hulls of a double hull structure when a joint member, a holder or a through member are inserted into a hull having the double structure of a small planing boat and improve the appearance of the planing boat.

CONSTITUTION: A joint member 40 is attached in such a manner that it passes through a double structure hull H provided with an outer hull 20 and an inner hull 30 which are molded by using molds. The joint member 40 is bonded to an inner face of the inner hull 30 by a collar 42, and a cap 43 is mounted on outer periphery of a projection section 40a to the outside of a hull to bring the cap 43 into contact with an outer face of the outer hull 20. An adhesive A is held among the cap 43, the projection section 40a, and the outer hull 20. In particular, the leakage of water from the outside of the hull is effectively prevented by the adhesive A among them so that water does not leak into a space between the outer and inner hulls 20, 30. Moreover, since the adhesive A is covered by the cap 43 so that it is not exposed to the outside of the hull, the appearance of a small planing boat is improved.