A 13-year-old girl with mixed connective tissue disease (MCTD) was described. She visited our hospital with recurrent parotid gland swelling, arthritis, and myositis. Sclerodactyly and Raynaud's phenomenon were also defined, and the laboratory findings of high titers of antinuclear antibody (speckled type), positive anti-RNP antibody, positive rheumatoid factor, and hypergammaglobulinemia suggested the diagnosis of MCTD associated with Sjögren syndrome. The muscle weakness and the increased levels of CK prompted us to examine the muscle biopsy and to perform the electromyography, both of which suggested severe muscle inflammation. The siarography and lip biopsy indicated definitively the association of Sjögren syndrome. Corticosteroid therapy including methyl-prednisolone pulses was started, but the effects were limited. The addition of low-dose methotrexate effectively lowered the levels of CK, and gradually improved the muscle strength. Thus, low-dose methotrexate therapy is recommended to the patients with MCTD who have severe myositis refractory to corticosteroid.