We conducted a seroepidemiological study in a northern Japanese prefecture where the incidence of identified COVID-19 cases was low. In June 2020, residual sera from 1009 outpatients were tested for antibody to SARS-CoV-2 by electrochemiluminescence immunoassay. Five specimens (0.5%) tested positive, suggesting low prevalence of SARS-CoV-2 infections in this population.