Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) has been affecting Hokkaido, Japan since late February 2020 until present. The aim of this study was to report the relationship between anti-SARS-CoV-2 antibody-positive and SARS-CoV-2 PCR-positive cases by analyzing anti-SARS-CoV-2 antibodies (IgG and total-Ig).Serum samples were collected from care workers and nurses in two nursing homes and two hospitals which underwent virus outbreak. All people were confirmed to be SARS-CoV-2-positive by RT-qPCR and their sera was analyzed for anti-SARS-CoV-2 antibodies (IgG and total-Ig).Although 34 out of 43 samples (79.1%) showed enough amount of anti-SARS-CoV-2 antibodies, 9 RT-qPCR -positive samples (20.9%) showed absence of anti-SARS-CoV-2 antibodies in their sera.The results that 20.9% of RT-qPCR-positive samples with SARS-CoV-2 showed absence of anti-SARS-CoV-2 antibodies provides a possibility that the innate immune reaction could eliminate the virus without activating adaptive immune reaction.