Our objective was to elucidate the genetic epidemiology of familial amyotrophic lateral sclerosis (FALS) and sporadic ALS (SALS) with OPTN mutations in the Japanese population. Mutational analysis of OPTN was conducted in 18 FALS pedigrees in whom mutations in other causative genes have been excluded and in 218 SALS patients by direct nucleotide sequence analysis. Novel non-synonymous variants identified in ALS patients were further screened in 271 controls. Results showed that although no mutations were identified in the FALS pedigrees, a novel heterozygous non-synonymous variant c.481G > A (p.V161M) was identified in one SALS patient, who originated from the southernmost part of the Kii Peninsula. The mutation was not present in 271 controls. As the clinical feature, the patient carrying V161M showed predominantly upper motor neuron signs with slow progression. This study suggests that mutations in OPTN are not the main cause of ALS in the Japanese population.