Although myocarditis following immune checkpoint inhibitor (ICI) therapy is rarely reported, it is considered clinically important because of its high mortality rate. Although various tests may be used for early diagnosis, abnormalities suggestive of myocarditis may not be detected. We report a case of ICI-induced myositis and concurrent asymptomatic myocarditis with mild cardiac marker elevation following nivolumab therapy in a 79-year-old man with metastatic gastric cancer. In this case, cardiac magnetic resonance imaging was useful for diagnosis. Treatment with oral prednisolone rapidly improved the patient's symptoms and creatine kinase levels. Follow-up examination revealed no flare-up of myositis and exacerbation of myocarditis. Since ICI-induced myositis is often complicated by myocarditis, this case report highlights the importance of detecting concurrent myocarditis in patients with ICI-induced myositis through intensive cardiac assessments to improve clinical outcomes.