This paper deals with the resonant type of wave energy converter (WEC) and the determination method of its geometric parameters, which were obtained to construct a robust and optimal structure. The optimization problem is formulated with the objectives of simultaneously maximizing the absorbed power output, which is mainly related to optimal power take-off damping, and minimizing the production cost by the volume of the required sheet plate using a weighting method. The constraints are composed of the response surfaces that indicate the resonance period (heave, pitch) and the meta-center height of the buoy. The signal-to-noise ratio calculated from the normalized multi-objective results with the weight factor can be used to help select the robust design level. In order to obtain a sample data set, the motion responses of the power buoy were analyzed using a commercial code based on the boundary element method. We present the Pareto-optimal set to reveal the relationship between the power and the volume of a sheet plate. Lastly, the power efficiency of the WEC with the optimum design variables is estimated as the captured wave ratio resulting from the absorbed power. The result of the WEC design is economically optimal and satisfies given constraints.