Introduction To determine basic patient characteristics and biomarkers to help in the early diagnosis of Coronavirus disease 2019 (COVID-19) pneumonia in patients with pneumonia who visited the emergency department (ED). Material & Method This retrospective study evaluated patients diagnosed with community-acquired pneumonia (CAP) or COVID-19 pneumonia in ED at four tertiary medical centers between February 1 and March 31, 2020. Parameters related to the differential diagnosis between CAP and COVID-19 were investigated. Clinical characteristics and laboratory results of biomarkers were analyzed. Result In total, 81 patients presented to the ED with COVID-19 pneumonia. Multivariate logistic regression analysis showed that monocyte count [odds ratio (OR): 0.996; 95% confidence interval (CI): 0.994-0.999] and pneumonia severity index (PSI) [OR: 1.025; 95% CI: 1.002-1.049] were associated with diagnosis of COVID-19 pneumonia. The area under the curve comprising the combination of monocyte and PSI was 0.789. Conclusion Differential diagnosis of COVID-19 pneumonia from pneumonia patients who visited the emergency room can be made by monocyte count and PSI score.