The coronavirus disease (COVID-19) is known to cause hyperferritinemia and haemophagocytic lymphohistiocytosis. Including this laboratory parameter, symptoms similar to COVID-19 have been observed in adult-onset Still's disease (AOSD), catastrophic antiphospholipid syndrome, macrophage activation syndrome, and septic shock, which has led to the proposal of a concept called 'hyperferritinemic syndromes'. High levels of some clinical markers in both COVID-19 and AOSD make them difficult to differentiate. While the efficacy of ciclesonide had been expected for mild pneumonia with COVID-19, the efficacy of tocilizumab (TCZ), which is a known treatment for AOSD, was not established. We report the first known occurrence of COVID-19 diagnosed in March 2020, preceded by the diagnosis of AOSD in April 2019. The patient was given prednisolone and TCZ, which led to remission. With the dyspnea and ground-glass appearance on chest computed tomography, PCR test revealed COVID-19 infection. Ciclesonide was started on Day 7 of the disease onset, which led to improved inflammatory markers. We infer that while TCZ is theoretically useful for COVID-19 due to its inhibition of interleukin 6. AOSD and COVID-19 may be differentiated by levels of ferritin, and appropriate treatment must be allocated.