An gene was identified as a RA disease susceptibility gene on Human Chromosome 8, the gene coding a protein that has an amino acid sequence shown in SEQ. ID NO. 1 and that has such mutation that glycine is inserted as a 269th amino acid in the sequence. Moreover, it was found that mutation of the gene and the protein relate to onset of RA. Achieved is a method of evaluating with high accuracy the onset or onset possibility of RA by using the mutation.