A 29-year-old woman with benign congenital nemaline myopathy is reported. She did not walk until the age of one year and seven months. Although she acquired the ability to run, she ran very slowly. She first noticed the progression of weakness of the limbs at age 21, and it worsened gradually. On admission, she showed moderate weakness in the face, neck, and four limbs. Serum creatine kinase was elevated to 218 U/l. Needle electromyography showed giant and polyphasic motor unit potentials with a reduced reference pattern in the four limbs diffusely. In muscle biopsy, about 10% of fibers had many small vacuoles, and half of them were rimmed. Modified Gomori trichrome stain revealed nemaline rods in about 20% of both type I and type II fibers. Fibers with large diameter and atrophic ones showed increased acid phosphatase activity. Type I fibers were small, and type II fibers numbered only 2%. We diagnosed her illness as a congenital nemaline myopathy that began in infancy and progressed in adulthood. The increased autophagic activity probably caused the progression of muscle weakness. Moreover, the presence of both nemaline rods and rimmed vacuoles may have contributed to the development of diffuse neurogenic changes seen in electromyography.