To enhance a reinforcing increase effect by externally fitting metallic groove shape fixtures or metallic fixtures with an H shape cross section to a rib.

The rib 11 is projectingly arranged on the rear side surface of the bottom board 1 of an FRP bathtub. A slope 21 and a pulling-out preventing projection 22 are disposed in opening part of the metallic groove shape fixtures 2. The fixtures 2 are externally fitted to the rib 11, the projection 22 is engaged with the side surface of the rib 11 and the fixtures 2 are prevented from easily coming off the rib 11. Thus, the fixtures 2 are externally fitted to the rib 11 so that the bottom board 11 of the bathtub is sufficiently reinforced. Besides, the slope 21 is arranged in opening part of the fixtures 2 so that the fixtures 2 are easily externally fitted to the rib 11. Moreover, the pulling-out preventive projection 22 is arranged in the opening part of the fixtures 2 so that the projection 22 is engaged with the side surface of the rib 11 to prevent the fixtures 2 from easily coming off the rib 11.