A 43-year-old woman was admitted to our hospital for myalgic pain and erythema of the face and thighs. She had been diagnosed with dermatomyositis at another hospital 8 years earlier and proteinuria had developed suddenly. Renal histological examination revealed membranous nephropathy. Membranous nephropathy associated with dermatomyositis is very rare. To date, malignant disease has not been detected in our patient, but the serum level of CA125, a tumor marker, has been elevated to 1,097 U/ml. The increased CA125 level is related to the degree of proteinuria and after prednisolone and cyclosporine therapy, proteinuria and the CA125 level gradually decreased.