The numerical methods for analyzing a terminator type wave energy conversion device are described under the condition that the linear water wave theory is applicable. Two methods are proposed to calculate the device characteristics of an Oscillating Water Column (OWC) type. One uses the flow rate and the gauge pressure in the air chamber directly, because in this system the interaction between the OWC and the turbine is found to be controlled only by the flow rate and the pressure drop through the turbine. The other method uses the equation of the floating body motion in a manner similar to the equivalent floating body approximation. The relations between these two methods are also examined. The hydrodynamic performance with frequency from zero to infinity is required in the simulation of irregular waves. However, resonance occurs when the air chamber breadth is equal to multiples of half a wavelength. Finally, it is confirmed that these solutions give good agreement with the experimental results.