The attenuation effect of barrier materials, which covers an explosive completely, on blast waves was studied. The density of the barrier materials was examined to make the barrier materials light and low in volume. Water gel, small spheres of foam polystyrene, and mixtures of these two materials were used as the barrier materials, and the density of the mixture was varied from 0.12 g•cm-3 to 1.0 g•cm-3 by changing the mixed volume ratio. Natural silica sand was also tested for comparison. A spherical PMMA container was filled with the barrier materials and a spherical pentolite (100 g) was ignited at the center of container. The blast pressure around the container was measured. The mixture of the density of approximately 0.55 g•cm-3 maximized the attenuation of the blast wave for the same volume. The attenuation effect depends not only on the weight of the barrier materials but also on the porosity. A mixture of a density of approximately 0.13 g•cm-3 maximized the attenuation of the blast wave for the same weight. Using porous materials, relatively light barrier materials can attenuate the blast wave effectively, if the volume is not restricted. The attenuation effect of sand was greater than that of water gel and a mixture for the same volume.