PURPOSE: To prevent the intrusion of water through an inspection hole by forming the circumferential section of the inspection hole opening to the extention of a deck at the rear of a transom with a bent edge section of a lower layer board while sealing between said edge section and the upper layer board end section with FRP sealing material.

CONSTITUTION: An inspection hole or an opening 6 is formed through an extention 2 of a deck at the rear of a transom 4, then an upper layer board 21 having FRP lower layer board 22 and a core material is laminated on the extention 2 and adhered by an adhesives 23. The lower layer board 22 is molded into a reverse U-shape by the circumferential edge of the opening 6 and formed with an edge section 7, while an upper layer board 21 is arranged only at the outside of said edge section 7 and the gap between said edge section 7 and the end section of the upper layer board 21 is sealed by an FRP sealing board 8 sticking over them. Consequently said sealing can be performed by only sticking a sealing material 8 on a plane to improve the workability resulting in the reliable adhesion.