The Kaimei is a wave power absorber developed as a large scale device at the Japan Marine Science and Technology Center. This device is a long floating body and has many air-chambers over which are arranged the air-turbine generators. The Kaimei is specified as an attenuator and a water column oscillator because it is moored in the head sea and the wave power is gradually absorbed through the rise and fall of the water in each air-chamber. In this paper the authors investigate the optimum design procedure of the Kaimei type wave power absorber. The optimum design means here maximizing the absorbable wave power by adjusting the external load distribution under the prescribed design period. The Kaimei is modelled as the long floating boxes with different arrangement of air chambers. Two types of ships, that is, a catamaran buoyancy room type and a separation buoyancy room type are mainly examined. It is assumed that the water in the air chamber is treated as a column of constant mass. The hydrodynamic forces are estimated by the Green-function integral equations technique. The following results are concluded. (i) In estimating the performance of the converter, a simple method which assumes the uniform load distribution over all the air chambers is useful as the practical design procedure. (ii) The performance is preferably insensitive to the variation of the external load condition around the optimum value. (iii) The design period should be long enough because both of the frequency response characteristics and of the efficiency. (iv) The separation type excels the catamaran type in wave power absorbability, and the performance of both types can be improved when the ship is free at some design period. (v) The Kaimei is the middle configuration of the above two types, but the performance tends to reduce to each one.