OWC (Oscillating Water Column) type wave energy converter installed in a breakwater has been investigated in Spain, Portugal, Australia and Japan. The OWC unit consists of a curtain wall, an air chamber, an air turbine and a generator. An air chamber has an opening in underwater part, and a wave goes in and out through this opening. The water surface compresses the air in an air chamber, and jet is discharged to turbine through a hole attached to the top of the air chamber. In our system, impulse turbine is applied to avoid performance decrease of wells turbine in stall condition. To rotate in one direction in an oscillating flow, guide vanes are attached. The arrangement of the guide vanes and turbines are optimized in a wind tunnel tests. Primary conversion efficiency is measured in a water channel prior to installation of turbine. In this paper, the theoretical study of the total conversion process of an OWC type wave energy converter which has a turbine is investigated. And the design method of a wave energy converter is shown and validated using an experiment results.