From December 26, 2020 to February 28, 2021, quantitative antigen test (Ag test) of SARS-CoV-2 was conducted by using a nasopharyngeal swab for 990 patients aged from 0 to 99 years old (average age; 63.1 +/-22.5 years old, male: 537, female: 463), who presented at least one of the following symptoms, such as fever, respiratory symptoms, and taste disorder. In 578 patients, Ag test and PCR test were conducted at the same time and these patients were analyzed. In 74 patients with Ag > = 1.0, 44

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... tients were PCR positive (positive ratio = 0.59). In 43 patients with Ag > = 10.0, 41 patients were PCR positive (positive ratio = 0.95), and all of 42 patients with Ag > = 100.0 were PCR positive. In 504 patients with Ag < 1.0, only five patients were PCR positive (positive ratio = 0.01). In 31 patients with 10 > Ag > = 1.0, three patients were PCR positive (positive ratio = 0.10). All eight patients presented a typical pneumonia, and five patients had a history of close contact with COVID-19 patients within 14 days. With an appropriate cut off value, Ag test is concluded to have enough sensitivity and specificity. Considering the advantage of handiness, rapidness, high-throughput and economy over PCR test, Ag test seems useful diagnostic method for COVID-19 in the pandemic situation.