Abstract: A resonant type apparatus for absorbing wave energy arranged at wave-breaking facilities, which comprises, a caisson having a bottom plate, side plates, a backside plate and a top plate at least a part of which is omitted. A water chamber in the caisson has a length in a directional parallel to the side walls which is larger than 1/4 of the wave length Lc of a stationary wave within the water chamber, and a node of the stationary wave in the water chamber is formed at a distance Lc/4 from the backside plate. A pendulum is arranged at the position of the node of the stationary wave for swinging with a natural period Tp which is substantially the same value as the natural period Tw of the stationary wave, whereby the pendulum is swung by the stationary wave to absorb and convert wave energy to useful available energy at high efficiency.