In this paper the authors investigate the wave-power absorbing performance of ship utilizing the anti-rolling tank (A. R. T.). In the first part, an ordinary-ship model is examined on the performance both by the experiments and by the theoretical calculations. The wave power absorbability is, however, found to be poor, but the design procedure is proved to be useful. In the second part, the effect of the floating-body shape on the performance is investigated basically, which shows that the deep draft and low center-of-gravity type is advantageous. Its remarkable characteristic is the easy adaptability to the variation of the sea condition by changing the c. g. position. This is also proved by the experiments. Finally, the basic design of the A. R. T. wave power absorber is proposed and estimated. The theoretical evaluation shows that the significant mass of tank water makes the wave-power absorbing response effectively wide by shifting the A. R. T. up and/or down according to the sea condition.