A 75-year-old man had been given a diagnosis of small cell lung cancer (SCLC) in March 2005. He had been treated with chemotherapy consisting of carboplatin and etoposide, and he had showed partial response to treatment. He presented with general fatigue, weakness of the muscles and extremities, skin lesions and finger erythema in April 2006. A muscle biopsy was performed and the pathological findings confirmed dermatomyositis. The findings of his chest X-ray and computed tomography showed an increase in the size of the lung cancer lesion. The tumor size decreased, and creatine kinase and lactate dehydrogenase levels also decreased after irinotecan chemotherapy. He was also given prednisolone (1 mg/kg) for prolonged muscle weakness. There are few cases which report dermatomyositis after a diagnosis of SCLC. In the present case, dermatomyositis appeared when the recurrence of SCLC became evident. We believed that the onset of dermatomyositis might be related to the activity level of SCLC.