This paper describes the calculation method of shear force and bending moment induced on pontoon type floating structures in random sea in taking account of motions of the floating structures, and the results of the comparison between the presented calculation method and those formulae by Muller which is deduced from equilibrium condition of dead weight and the buoyancy of the floating structure, Standard for prestressed concrete barge and the Veritus Rule.
　The present calcualation method is established by means of approximate solution of motions for two dimensional floating structures, and the procedure of calculation is explained so carefully that it become easier for any designer to use it.
　In the comparison and the consideration, effects of such items as motions, water depth ,irregularity of waves which are not included in previously used formulae. And it concluded that it is recommended to used the presented method especially for the case in which the water depth is shallow and the wave length is larger than the length of the floating structures.