Provided is a protein aggregation inhibitor capable of inhibiting aggregation of a protein even when the protein is heated, and further, capable of inhibiting a decrease in the protein activity even when the protein is heated, which characteristically contains a sulfobetaine polymer obtained by polymerizing monomer components containing sulfobetaine monomer represented by the formula (I):wherein R1 is a hydrogen atom or a methyl group, R2 is an alkylene group having a carbon number of 1 - 4, R3 is an alkyl group having a carbon number of 1 - 4, R4 is an alkylene group having a carbon number of 1 - 4, and X is an -NH- group or an - O- group.
