Copolymer containing functional groups such as polyethylene oxide (PEO) and maleic anhydride (MA) was synthesized to modify the cellulase. MA was attached to PEO allyl ester, which was the product of 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. MA groups of the copolymer formed the chemical bond with the amino acid groups of the cellulase in the modification reaction. When cellulase was modified with synthesized copolymer, modified enzyme showed high relative activity regardless of high degree of modification compared with other modification methods. In mixed office waste reprocessing, modified cellulase improved many physical properties of the paper including freeness, optical properties, and strengths compared to the conventional process. Even though native cellulase improved the physical properties, modified cellulase showed an increased tensile strength and internal bond over those of unmodified cellulase. From these results, modified cellulase method is an effective biological treatment that would save pulp resources in the reprocessing.