A 78 year-old woman was admitted to our hospital because of subacutely progressive dysarthria, dysphagia, proximally dominant muscle weakness and erythema in the neck and back. She was diagnosed as having rheumatoid arthritis (RA) at the age of 60 and treated with bucillamine (BUC) for 8 years. Laboratory tests included a rheumatoid factor of 1,472U/ml. Serum creatine kinase level was slightly elevated. The activated T cells in the peripheral blood were markedly increased. Needle EMG demonstrated myogenic changes. The magnetic resonance image of the left upper arm showed diffuse muscle atrophy and inflammatory changes in the triceps muscle. The muscle biopsy revealed perivascular inflammatory cell infiltraton and type II fiber atrophy. A biopsy from the skin showed mild perivascular inflammatory cell infiltraton. According to the results of these findings, she was thought to have dermatomyositis due to BUC. After withdrawal of BUC followed by the administration of prednisolone 1mg/kg, her symptoms improved and activated T cells in the peripheral blood were decreased. In Japan, BUC is widely accepted as an effective drug in the treatment of RA, even though it is known to induce some autoimmune diseases. However, the mechanism of the development of autoimmune disease is unclear. We considered that the long-term use of bucillamine could trigger an autoimmune response such as an increase in activated T cells and the development of dermatomyositis-like clinical features in our patient. In conclusion, when RA patients treated with BUC show a clinical picture compatible with dermatomyositis, its causative relationship has to be considered.