The system simulation for a fixed O. W. C. type wave absorber is proposed, which takes into consideration not only the interaction between the O. W. C. and the turbine but also the effects of air compressibility in the air-chamber. Linear calculations for steady high velocity revolution in frequency domain are also shown. It is shown that the reactance due to the air compressibility is as large as restoring force due to static pressure of the water column in case of real sea trial.