Canine degenerative myelopathy is an adult-onset, progressive neurodegenerative disease that occurs in multiple dog breeds, particularly Pembroke Welsh Corgis. Recently, a degenerative myelopathy-associated mutation of the canine SOD1 gene was identified as c.118G>A (p.E40K). In the present study, genotyping assays using conventional and real-time PCR methods were developed, and a preliminary genotyping survey was performed on 122 randomly selected Pembroke Welsh Corgis without any degenerative myelopathy-related clinical signs to determine the current allele frequency in Japan. Both of the assays provided clear-cut genotyping. The survey demonstrated the frequencies of the G/G wild-type, G/A heterozygote and A/A homozygote to be 9.0, 42.6 and 48.4%, respectively, indicating that the prevalence of the mutant A allele (69.7%) in Pembroke Welsh Corgis is extremely high in Japan.