We experienced a 72-year-old man who developed laboratory-confirmed human coronavirus HKU1 pneumonia. PCR testing for SARS-CoV-2 from a nasopharyngeal specimen was negative twice, and rapid immunochromatographic antibody test (RIAT) using a commercially available kit for IgM and IgG against SARS-CoV-2 showed him turning positive for IgG against SARS-CoV-2. We then performed RIAT in stored serum samples from other patients who suffered laboratory-confirmed human common cold coronaviruses (n = 6) and viruses other than coronavirus (influenza virus, n = 3; rhinovirus, n = 3; metapneumovirus, n = 1; adenovirus, n = 1) admitted until January 2019. Including the present case, four of 7 (57%) showed false-positive RIAT results due to human common cold coronaviruses infection. Two of the 4 patients showed initial negative to subsequent positive RIAT results, indicating seroconversion. RIAT was positive for IgG and IgM in viruses other than coronavirus in 2 (25.0%) and 1 (12.5%) patient. Because of high incidence of false positive RIAT results, cross antigenicity between human common cold coronaviruses and SARS-CoV-2 can be considered. Results of RIAT should be interpreted in light of epidemics of human common cold coronaviruses infection. Prevalence of past SARS-CoV-2 infection may be overestimated due to high incidence of false-positive RIAT results.