Floating devices, such as a cavity resonance device, take advantage of both the water motion and the wave induced motions of the floating body itself. In the design of a wave energy converter (WEC), the most significant factor is that an optimum length of the internal water column should exists, that is, a length in which maximum power is converted near the heaving resonance. A theoretical analysis of this power generated by a pneumatic-type WEC is performed, and the results obtained from the analysis are used for a real WEC for a buoy. The length of the internal water column corresponds to that of the water mass in the water column. If designed properly, a WEC can take advantage not only of the cavity resonance but also of the heaving motion of the buoy. This paper presents the test results of the generation characteristic of a WEC and the harmful effect of fouling in the internal water column. The results are then applied to the design of a WEC for a buoy.