The effect of the reduction in specific surface area and pore size distribution on the gas permeability has been studied in porous polypropylene film with sputtered metal coating.The results are as follows; 1) there is an empirical relation 1/Q=a+b/Δp between flux Q and difference pressure Δp in a porous membrane, 2) there exists a limit of flux Q in porous membrane and this shows drastic decrease with reduction in specific surface area, 3) pore size distribution changes remarkably with increasing the thickness of metal coating film on porous membrane, and 4) especially the coating effect appears in the pore size range from 20A to 80A.It was found that the gas permeability is affected largely by the change of pore size distribution.