“Mighty Whale” is an offshore floating type wave power device based on the oscillating water column (OWC) principle. It converts wave energy into electric energy, and produces a relatively calm sea area behind it. These electric energy and calm sea area will be able to utilize for varied applications such as to improve a deteriorated ocean environment and fish farming. JAMSTEC constructed the prototype of Mighty Whale for open sea tests by May 1998, and the open sea tests were started on September 1998 at the mouth of Gokasho Bay in Mie prefecture. This paper describes the characteristics of wave energy absorption of the prototype by the open sea tests data. Particularly the results of the prediction method of electrical output and water tank experiments were compared with open sea tests data on the output of air chamber, turbine output and generated output. The outline of the discussion about these results, the validity of prediction method of generated output was confirmed. And we recognized characteristics of wave energy absorption and wave energy efficiency of floating type wave power device. Based these results, the validity of the design for the prototype was confirmed.