A long caisson is proposed to reduce wave force and construction work. The long caisson is subjected to torsion due to wave force during towing, un-uniform settlement of baseground and so on. This caisson is fabricated by reinforced concrete or hybrid members. Mechanical properties of long caisson with open section at upper part due to torsion are not well known. The static loading test for three specimens with 5.3m length is carried out to know these mechanical properties. Ultimate strength, rigid deformation and cracks of test specimens are investigated. Theoretical equation considering warping torsion and St.Venant torsion is derived and validity of this equation is confirmed through comparison between test results and calculation results.