Independent Administrative Institution, Japan Agency for Marine-Earth Science and Technology (JAMSTEC) has been engaged in research and development of ocean-wave energy extraction technology for many years now. In particular, work began in 1987 an offshore floating type wave power device called “Mighty Whale”. Theoretical investigations and model tests led to an understanding of the hydrodynamic behavior of device, and provided information of constructing for the prototype of Mighty Whale. The open sea tests were started on the 10th of September 1998. Tests were conducted until March 2002. In this paper, firstly, the characteristics of hydrodynamics with “Mighty Whale” have been examined comparing with the results of experiments and calculations approving a numerical method being developed. Secondly, using developed numerical analyze method, the displacements of water level inside air chambers of “Mighty Whale” have been calculated. The generating outputs of “Mighty Whale” have been estimated using these results, the efficiency of primary conversion and the efficiency of secondary conversion of “Mighty Whale” getting through the experiments that had been carried out.