A water swelling material is one of the rubbery impermeable materials which mixed synthetic resin elastomers as a base material, high absorbency polymers, filler and solvents. In this study, swelling characteristics of the water swelling material on the water polluted with COD and BOD, as an impermeable material at coastal waste landfill site, are examined by laboratory swelling ratio test. Furthermore, the factor in which it influences the swelling pressure of water swelling material is clarified by measuring the swelling pressure. As the results, the COD nor the BOD concentrations in the soaked water influence the swelling ratio of the water swelling material. When the thicknesses of water swelling material are 2 and 3mm, the maximum swelling pressure of 0.5 and 0.7MPa that correspond to hydraulic pressure by depth of 50 and 70 m is possessed, respectively.