Infectious disease outbreaks have a great influence on the use of emergency departments (EDs). The purpose of this study was to analyze the effect of the early coronavirus disease outbreak (COVID-19) on ED visits, to be able to more effectively use ED resources during infectious disease outbreaks. This was a retrospective observational study conducted in three tertiary EDs. We defined the COVID-19 period as 01 December 2019, to 30 April 2020, and the control periods were defined as the same period in 2018 and 2019. All patients who visited the EDs during the study period were included. We collected visitor data on the total number, demographics, clinical data, vital signs, acuity level, cause of visit, disposition, time of visit and diagnostic code. A total of 180,192 patients were enrolled in this study. During the COVID-19 period, the number of ED visits decreased significantly. This decline was significant since mid-February when the number of COVID-19 patients surged nationwide. The proportion of critical patients and admission rate increased while the proportion of non-emergency patients decreased significantly from February (p < 0.001). The proportion of night-time ED visits decreased significantly from March. During the COVID-19 period and pre-COVID-19 period, the top three diagnosis areas were “abdominal and pelvis pain” “fever of other and unknown origin” and “open wound in the head” and remained unchanged. “Influenza because of an identified seasonal influenza virus” decreased during the study period. The study showed the number and characteristics of ED visits changed during the COVID-19 period compared with those of pre-COVID-19 periods.