Copolymer containing functional groups such as polyethylene oxide (PEO) and maleic acid anhydride (MA) was synthesized to modify cellulase. MA was attached to the PEO allyl ester, which was the product formed by the reaction between PEO allyl alcohol and lauric acid. The number of ethylene oxide (EO) units in one PEO chain was varied from 10 to 40, and MA formed the chemical bond with the amino acid groups of the cellulase for the modification reaction. When cellulase was modified with synthesized copolymer, activity of the modified cellulase decreased slightly as the degree of modification increased. The modified enzyme showed high remaining activity regardless of a high degree of modification. At the maximum modification degree of 52%, the modified cellulase activity retained more than 65% of the unmodified native cellulase. Modified cellulase retained higher reactivity than native cellulase in an organic solvent and at various pH values,