PURPOSE: To prevent a water leak from the fitting positions of handhold metals by extending an architrave on the periphery of an upper opening section, providing a backrest section on one side of a bathtub, and integrally forming the handhold metals protruded inward on the bathtub through both side walls across the backrest section.

CONSTITUTION: An architrave 11 is extended over the whole periphery of an upper opening section, a backrest section 12 is provided on one side of an FRP bathtub 1, and a pair of handhold metals 2 are protruded inward face to face on the bathtub 1. The handhold metals 2 are made of stainless steel, formed into a nearly bottomed elliptical cylindrical shape, and integrally molded through both side walls 13 across the backrest section 12 of the bathtub 2 while their bottom sections are faced inward. The handhold metals 2 protruded inward are formed on the bathtub 1, post-machining such as boring which may cause a leak on the bathtub 1 is not required, and the airtight bathtub 1 is obtained.