As anti-RNA polymerase (RNAP) antibodies are detected with high frequency in patients suffering from cutaneous scleroderma where skin sclerosis progresses rapidly, supervenes scleroderma renal crisis at a high rate, and associates with clinical entities whose prognoses are extremely bad, it is intended to provide a convenient method of detecting an anti-RNAP antibodies, which is extremely useful in diagnosing and classifying clinical entities of scleroderma, and predicting organ failure, in particular scleroderma renal crisis. In order to identify an epitope recognized commonly by anti-RNAP antibodies, the full length of RPC62 and a partial fragment of RPC155, that are 2 subunits of 62-kDa and 155-kDa of RNAP III, are expressed in Escherichia coli as recombinant proteins, and the reactivities to sera positive and negative to anti-RNAP antibody from patients suffering from scleroderma are examined by immunoblotting method to confirm that an epitope recognized by anti-RNAP antibodies in the sera from the patients suffering from scleroderma exists in 891 to 1020 amino acid residues of RPC155.