In recent years, ocean exploration and development activities have moved into deeper and more exposed seas, with progress of offshore technology. In these offshore construction works, it is very important to minimize the downtime of work vessels which have special functions, such as crane, pipe laying, diving support, etc.. Therefore, the reduction of motions of such vessels and floating structures has much significance on progress of offshore projects. In order to reduce the motions of floating structures, the effectiveness of new motion suppression system is dealt with based on modified Bell's type tanks which are composed of air chamber, water columns opened to sea and air duct between tanks. In this paper, as fundamental study, hydrodynamic forces acting on rectangular barge and its water columns in tanks are calculated by two-dimensional singular distribution method. Then, after modelization of air flow in chamber and duct, motion equations are derived. Results of calculation are compared with the experimental data. The results of motion calculation agree well with experimental ones. The results of this study are following. (1) The main factor of effects of this motion suppression system is shift of resonant period of structure to longer due to reduced GM . (2) The magnitude of rolling amplitude of structure with motion suppression system at its resonant period becomes abt.1/5 compared with amplitude without suppression system. (3) By handling of valve in air duct, very high performance of motion characteristic of structure on overall range of frequency is obtained.