A copolymer for suppressing protein adsorption, wherein the copolymer has a structure of a block copolymer or graft copolymer, which has a polymer chain of a constitutional unit (a), where the polymer chain is represented by Formula; and a polymer chain of a constitutional unit (b), where the polymer chain is represented by Formula, in the formula, R 1 represents a hydrogen atom or a methyl group, R 2 is an OR 33, a halogen atom, COR 34, COOR 35, CN, CONR 36 R 37, or R 38, where R 33 to R 37 each independently represent a hydrogen atom, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alicyclic group, a substituted or unsubstituted aryl group, a substituted or unsubstituted heteroaryl group, a substituted or unsubstituted non-aromatic heterocyclic group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted alkaryl group, or a substituted or unsubstituted organosilyl group, and R 38 represents a substituted or unsubstituted aryl group or a substituted or unsubstituted heteroaryl group, in the formula, R 3 represents a hydrogen atom or a methyl group, R 4 represents an alkylene group having 1 to 4 carbon atoms, and R 5 represents a hydrocarbon group having 1 to 6 carbon atoms, wherein n is a natural number of 1 to 1,000,000 and m is a natural number of 1 to 1,000,000, and wherein the copolymer has a weight-average molecular weight (Mw) of 75,000 or more, as measured using gel permeation chromatography as defined in the description. a polymer chain of a constitutional unit (a), where the polymer chain is represented by Formula; and a polymer chain of a constitutional unit (b), where the polymer chain is represented by Formula, in the formula, R 1 represents a hydrogen atom or a methyl group, R 2 is an OR 33, a halogen atom, COR 34, COOR 35, CN, CONR 36 R 37, or R 38, where R 33 to R 37 each independently represent a hydrogen atom, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alicyclic group, a substituted or unsubstituted aryl group, a substituted or unsubstituted heteroaryl group, a substituted or unsubstituted non-aromatic heterocyclic group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted alkaryl group, or a substituted or unsubstituted organosilyl group, and R 38 represents a substituted or unsubstituted aryl group or a substituted or unsubstituted heteroaryl group, in the formula, R 3 represents a hydrogen atom or a methyl group, R 4 represents an alkylene group having 1 to 4 carbon atoms, and R 5 represents a hydrocarbon group having 1 to 6 carbon atoms, wherein n is a natural number of 1 to 1,000,000 and m is a natural number of 1 to 1,000,000, and wherein the copolymer has a weight-average molecular weight (Mw) of 75,000 or more, as measured using gel permeation chromatography as defined in the description. in the formula, R 1 represents a hydrogen atom or a methyl group, R 2 is an OR 33, a halogen atom, COR 34, COOR 35, CN, CONR 36 R 37, or R 38, where R 33 to R 37 each independently represent a hydrogen atom, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alicyclic group, a substituted or unsubstituted aryl group, a substituted or unsubstituted heteroaryl group, a substituted or unsubstituted non-aromatic heterocyclic group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted alkaryl group, or a substituted or unsubstituted organosilyl group, and R 38 represents a substituted or unsubstituted aryl group or a substituted or unsubstituted heteroaryl group, in the formula, R 3 represents a hydrogen atom or a methyl group, R 4 represents an alkylene group having 1 to 4 carbon atoms, and R 5 represents a hydrocarbon group having 1 to 6 carbon atoms, wherein n is a natural number of 1 to 1,000,000 and m is a natural number of 1 to 1,000,000, and wherein the copolymer has a weight-average molecular weight (Mw) of 75,000 or more, as measured using gel permeation chromatography as defined in the description.