To obtain a polypeptide comprising a specific amino acid sequence, having antigen activity against anti-aldolase A antibody, capable of suppressing nonspecific reaction, having extremely low incidence of pseudopositive reaction and enabling diagnosis of chronic rheumatoid arthritis having high accuracy.

This polypeptide comprises an amino acid sequence of the formula or an amino acid sequence in which one amino acid or plural amino acids are added, inserted, deleted or substituted in the amino acid sequence of the formula and has antigen activity against anti-aldolase A antibody. Furthermore, it is preferable to prepare this immune reagent for diagnosis of chronic rheumatoid arthritis comprising an antigen polypeptide against an anti-aldolase A antibody comprising the polypeptide and can detect the antialdolase A antibody in a specimen by antigen antibody reaction and the immune reagent preferably comprises a fused protein of the antigen polypeptide with glutathione-S- transferase.