A 38-year-old woman had a history of asthma for 20 years. Bullous lesions had appeared in her left side of the back. Two months before admission, and biopsy revealed eosinophilic cellulitis. She experienced numbness in both legs one month later. She was admitted to our hospital for emergency treatment due to chest pain and loss of consciousness. Emergency coronary angiography revealed triple-vessel vasospasm. She had cardiac arrest for four minutes during the examination. We suspected eosinophilic granulomatosis with polyangiitis due to pulmonary infiltrate, eosinophilia, and a history of illness. We therefore started methylprednisolone pulse therapy. Although her condition and laboratory findings improved, cardiac magnetic resonance (CMR) imaging performed on day 16 showed myocardial edema and myocardial fibrosis on late gadolinium enhancement (LGE). Coronary angiography on day 35 revealed no spasm, and myocardial biopsy showed absence of vasculitis. There was no improvement in myocardial edema. CMR showed enlargement of LGE and formation of ventricular aneurysm. As myocarditis did not improve sufficiently, five courses of intravenous cyclophosphamide pulse therapy were administered. CMR on day 152 showed disappearance of the myocardial edema. We report a unique case of successful treatment of severe myocarditis and the usefulness of follow-up CMR.