The authors have been developing a floating WEC (wave energy conversion) system with multiple cylindrical OWC columns. This structure has multiple OWC devices for electric power generation and buoyancy columns under the deck structure. In the present study, we have examined the scale effect of OWC device, focusing on the air compressibility of air chamber in OWC device, and have constructed mechanical modelling for the air chamber mechanics based on the theory of air damper with a nozzle. The relation between the water elevation motion and dynamic pressure in the air chamber is derived. We have applied it to our numerical program to estimate the primary energy conversion efficiency of OWC devices for various sizes. It is shown that the air compressibility has to be considered in case of realistic large size OWC device.