Hydraulic properties of a row of closely spaced circular piles as a breakwater have been studied both theoretically and experimentally. A theory is presented for the transmission of waves past the breakwater and also for the thrust and bending moment to be exerted by the waves upon each pile in the breakwater. Laboratory experiment has been made on a model structure. A pretty close agreement is shown in the comparison between the theory and the experiment with respect to the transmission coefficient and the bending moment distribution. Special emphasis is laid on the remarkable rate of decrease of the thrust and bending moment to be exerted on each pile in the breakwater with the increase of the space of the piles. In taking this economical aspect of this structure into consideration, the closely spaced pxle breakwater has been concluded as a promising type of breakwater of comparatively light structure.