Disclosed is a prophylactic or therapeutic agent for rheumatoid arthritis or diseases caused by rheumatoid arthritis (rheumatoid arthritis-related diseases), which comprises HLA-G or a dimer thereof as an active ingredient. As the HLA-G, the following protein (a) or (b) are suitably used.(a) A protein consisting of the amino acid sequence of SEQ ID NO: 1(b) A protein consisting of the amino acid sequence of SEQ ID NO: 1 with the deletion, substitution, or addition of one or several amino acids excluding the amino acid at position 42, and having a binding activity for leukocyte Ig-like receptors and/or CD8
