Diclofenac sodium plaster

A diclofenac sodium plaster comprising a backing material and a paste spread on the backing material, said paste comprising diclofenac sodium, a penetration enhancer composed of 1-menthol and propylene glycol, and a hydrophilic base composed principally of a water-soluble polymer, wherein diclofenac sodium is contained in an amount of 0.5-20 wt. % based on the paste, wherein the water-soluble polymer is contained in an amount of 0.5-40 wt. % based on the paste, wherein propylene glycol is contained in an amount of 0.1-60 wt. % based on the paste, wherein 1-menthol is contained in an amount of 0.05-10 wt. % based on the paste, and wherein the backing material is a woven or non-woven fabric of rayon, polyester, polyolefin or polyurethane fibers, a polymer film, or a foamed sheet.