Diffusion of shocked gas flow by a high porosity porous elastic foam is investigated experimentally. We investigate unsteady process occurring when shock wave passing through porous material. Especially we compare two cases : In the non fixed foam condition, the down stream side of the foam is fixed to the rear end wall of the driven tube. In the fixed foam condition, the foam is also fixed to the side wall of the driven tube. Two kinds of porous materials are treated in both cases (high-porosity φg=0.98 and low density ρc≃27 kg/m3. One-dimensional experiments are conducted by using a shock tube (50 mm inner diameter and 6.4 m in length). The diameter of polyurethane foams are 49.7 mm with 60, 90 and 180 mm in length, respectively. In the fixed foam condition, the integrated impulse obtained at the rear end wall D, decreases down to about 74% of the non fixed foam condition, showing that the momentum loss is higher than the non fixed foam condition.