A double-cylindrical caisson is planned to construct in deep sea around Japan to create wide calm water region. Double vertical cylinders are subjected to in-plane force as well as outplane force due to wave and earth pressure. In-plane loading test are carried out to know the strength and crack properties of six reinforced concrete (RC) plates and six prestressed concrete (PC) plate. Two specimens have square opening in the concrete plate. The ultimate strength of them, which are subjected to pure tensile force from two directions, can be estimated by equations proposed by Baumann and Duchon through the comparision of test results and calculation results. Crack direction in the test specimens is almost perpendicular to the direction of principal stress. Cracks scatter at intervals of about 25 cm in RC plate and 15 cm in PC plate.
　When cracks occur, crak widths become large contrary to expectation. it is necessary to pay attention to the crack width of the concrete plate due to the pure tensile force.