Abstract: An improvement of a medium for electrophoresis which comprises a support of a plastic material and an aqueous polyacrylamide gel, wherein a resin layer comprises a resin having an oxygen permeability coefficient lower than that of the plastic material of the support is arranged betwen the support and the polyacrylamide gel. The resin preferably shows an oxygen permeability of not more than 15 cc/m.sup.2 .multidot.atm.multidot.day in the form of a resinous layer of 20 .mu.m thick, and exemplified by a vinylidene chloride-vinyl chloride copolymer, polyvinyl alcohol, polyacrylonitrile, polyvinylidene chloride, a vinyl alcohol-ethylene copolymer, a vinylidene chloride-acrylonitrile copolymer, and an acrylonitrile-methyl acrylate-butadiene copolymer.