Background: We evaluated the clinical performance of an immunochromatographic (IC) IgM/IgG antibody assay for severe acute respiratory syndrome coronavirus 2 (SARS-CoV2) and chest computed tomography (CT) for the diagnosis of Coronavirus disease 2019 (COVID-19). Methods: We examined 139 serum specimens collected from 112 patients with COVID-19 and 48 serum specimens collected from 48 non-COVID-19 patients. The presence of IgM/IgG antibody for SARS-CoV2 was determined using the One Step Novel Coronavirus (COVID-19) IgM/IgG Antibody Test. Chest CT was performed in COVID-19 patients on admission. Findings: Of the 139 COVID-19 serum specimens, IgM was detected in 27.8%, 48.0%, and 95.8% of the specimens collected within 1 week, 1-2 weeks, and >2 weeks after symptom onset and IgG was detected in 3.3%, 8.0%, and 62.5%, respectively. Among the 48 non-COVID-19 serum specimens, 1 generated a false-positive result for IgM. Thirty-eight of the 112 COVID-19 patients were asymptomatic, of whom 15 were positive for IgM, and 74 were symptomatic, of whom 22 were positive for IgM and 7 were positive for IgG. The diagnostic sensitivity of CT scan alone and in combination with the IC assay was 57.9 % (22/38) and 68.4% (26/38) for the asymptomatic patients and 74.3% (55/74) and 82.4% (61/74) for the symptomatic patients, respectively. Conclusion: The IC assay had low sensitivity during the early phase of infection, and thus IC assay alone is not recommended for initial diagnostic testing for COVID-19. If RT-qPCR is not available, the combination of chest CT and IC assay may be useful for diagnosing COVID-19.