Intravenous immunoglobulin (IVIG) therapy has been introduced to idiopathic dilated cardiomyopathy due to their antiviral and anti-inflammatory effects. But each study reported conflicting result and treatment regimen has not been clearly established. We experienced a case of 28-year-old woman with idiopathic dilated cardiomyopathy with severely depressed cardiac function. Its onset time was obvious within 1 month. Despite of conservative treatment of heart failure, sudden cardiac arrest was developed. We tried IVIG therapy, and her symptoms and cardiac function were improved after IVIG treatment.