There is provided a protein stabilizer for stably storing a protein in a solution. The protein may be an enzyme, an antibody, an enzyme-labeled antibody, or the like for a biochemical assay, etc. There is further provided a protein stabilization reagent comprising a protein-containing solution and the protein stabilizer dissolved therein. The protein stabilizer comprises a copolymer prepared by copolymerizing a monomer (a) of the following formula (1), a monomer (b) of the following formula (2), and a monomer (c) of the following formula (3).