The present invention provides a protein aggregation inhibitor for use in preventing aggregation of a protein, which contains a terminal sulfanyl group-containing sulfobetaine polymer having a repeat unit derived from a sulfobetaine monomer represented by the formula (II), and a noble metal particle, and in which the terminal sulfanyl group-containing sulfobetaine polymer is chemisorbed on the noble metal particle by the sulfanyl group (the groups in the following formula are as defined in the DESCRIPTION).