Bending test and lateral loading test of the box-type RC segment, with 9500mm external diameter, 1200mm wide, 530mm hight of main girdar, and 230mm thickness of skin plate, are conducted so as to determine the effective width of the skin plate. It is confirmed that whole skin plate works against bending because plane sections including skin plate before bending remain plane after bending (Bernoulli's principle). On the other hand, stress state of skin plate under the lateral load is found to be the same with the half-infinite 2-dimensional elastic plate subjected to in-plane varying lateral load, hence the effective width can be determined focucing on the stress state.