To provide a prophylactic/therapeutic agent for neurodegenerative diseases (such as polyglutamine diseases), the agent containing an HMGB family protein or a derivative thereof, such as a protein according any one of (a) and (b) below: (a) a protein having the amino acid sequence of SEQ ID NO: 2, 4, 6 or 8; and (b) a protein having an amino acid sequence resulting from deletion, substitution, addition or insertion of one or more amino acids in the amino acid sequence of SEQ ID NO: 2, 4, 6 or 8 and having binding activity to an abnormal polyglutamine protein produced in a neurodegenerative disease.