Biocatalysts such as cells and enzymes are immobilized in a polymer gel by forming a mixture containing a biocatalyst, an N,N-dialkylacrylamide monomer, a cationic acrylamide monomer and a water-soluble cross-linking monomer, and copolymerizing the monomers to produce a polymer gel entrapping the biocatalyst. Preferably, the N, N-dialkylacrylamide is N,N-dimethylacrylamide or N,N-diethylacrylamide in an amount of about 70 to about 99.8% by weight, the cationic acrylamide monomer is N,N-dimethylaminopropylacrylamide, N,N-dimethylaminopropylmethacrylamide, N,N-diethylaminopropylmethacrylamide, N,N-diethylaminopropylacrylamide and quaternary compounds thereof in a amount of about 0.1 to about 10% by weight and the cross-linking monomer is N,N'-methylenebisacrylamide, N,N'-methylenebismethacrylamide, N,N'-(1,2-dihydroxyethlene)bisacrylamide, 1,3-diacrylamide methyl-2-imidazolidone or diacrylamide methylene urea in an amount of about 0.1 to about 20% by weight. The polymer gel containing a biocatalyst has excellent storage stability and does not putrefy even after one month of storage at ordinary temperature.