The patient was a 58-year-old man diagnosed with nasopharyngeal carcinoma （T1N3M1, stage ⅣB）. He received 15 courses of pembrolizumab, an immune checkpoint inhibitor. Two months after treatment, he presented to the emergency department with complaints of nausea, vomiting, and fatigue. A chest computed tomography scan revealed pericardial effusion. The patient was hospitalized for suspected pericarditis or myocarditis due to immune-related adverse events （irAE）, and was started on 60mg of prednisolone. However, six hours after admission, he exhibited symptoms of seizures and deterioration in cardiac function. Therefore, a steroid pulse with 1,000mg of methylprednisolone was administered for 3 days. Subsequently, the prednisolone dose was tapered from 60mg, and he was discharged on hospital day 22. Although the fatality rate for irAE myocarditis is reported to be around 50％, in this case, early therapeutic intervention resulted in successful survival. Moreover, it has been reported that ICIs show higher antitumor efficacy in cases where irAE occurs. Also, in this case, both the primary and metastatic lesions have remained reduced in size.