This paper describes about both analysis of performance of wave power absorbers, and prediction of absorbed wave power. The authors clear up the diffinition of absorbed wave power spectrum, and calculate the expected value of absorbed wave power around Japan by making use of the data of long term wave frequency. Salters Duck is used as one of examples of wave power absorbers. The conclusions are as follows : 1) Absorbed wave power spectrum consists of two components, the one is the spectrum which indicates the constant component of absorbed wave power, and the other means the varying component. These spectra can be estimated theoretically by use of the present method. 2) The long term expected value of absorbed wave power around Japan correspons to about twenty or thirty percents of the power supply in Japan in 1975.