We report herein the case of a 70-year-old patient with elderly onset rheumatoid arthritis associated with severe muscle pain in shoulder and pelvic girdle. The patient revealed erosive polyarthritis with high titers of rheumatoid factor. Muscle pain started one month after the onset of rheumatoid arthritis followed by muscle weakness and muscle atrophy. Synovial effusion and edema in the soft tissue outside of the articular capsule in the knee joint were confirmed ultrasonographically. Administration of prednisolone at 20 mg/day dramatically abolished the muscular manifestations. The coexistence of an early stage of elderly onset rheumatoid arthritis and polymyalgia rheumatica was considered due to the presence of seropositive erosive arthritis and severe muscle manifestations at the same time.