Eosinophilic myocarditis is a rare form of myocardial inflammation that is characterized by the infiltration of eosinophilic cells into the myocardium. The clinical symptoms of eosinophilic myocarditis are similar to those of acute coronary syndrome, and eosinophilic myocarditis sometimes occurs in combination with bronchial asthma. We herein present a case of eosinophilic myocarditis in which additional time was required to make a definitive diagnosis because the patient received steroid therapy. The diagnosis of eosinophilic myocarditis is challenging, especially when a patient has other inflammatory diseases, such as bronchial asthma. We should pay attention to the possibility that steroid therapy may mask the presentation of eosinophilic myocarditis.