Amyotrophic lateral sclerosis (ALS) is a disorder with strong clinical and genetic heterogeneity, and its pathogenic mechanism has not been completely clarified. Proximal myopathy is rare in clinical manifestations of ALS. Here, we describe a 34-year-old woman with a 1-year history of symmetrical, proximal limb weakness, and muscle atrophy, with slow progression and no upper motor neuron (UMN) signs. The clinical phenotype was similar to myopathy and was initially misdiagnosed as proximal myopathy. Electromyography (EMG) and muscle and nerve biopsy were performed. The genomic DNA from the patient's peripheral blood lymphocytes was analyzed. The EMG and pathologic examinations revealed chronic neurogenic changes and mild mixed peripheral neuropathy. DNA analysis revealed a heterozygous missense mutation in exon 1 at codon 50 (c.50G>C) of SOD1, and a heterozygous missense mutation in exon 11 at codon 1013 (c.1013G>A) of CPT1C that has not been reported previously. The patient was diagnosed as familial ALS (FALS) type 1, and the patient had a family history of autosomal dominant (AD) pattern. This report expands the knowledge of the clinical phenotype of FALS. For patients with clinical manifestations mimicking proximal myopa- thy, the possibility of underlying ALS should be considered.