The patient was a 1-year-and-4-mo-old boy. He had drunk about 1 L of an isotonic drink for infants daily since about 10 mo after birth. He was examined by a local doctor due to anorexia and vomiting, found to have cardiomegaly, and transported to our hospital with suspected myocarditis. After admission, the patient showed polypnea, a decreased level of consciousness, and marked metabolic acidosis and lapsed into circulatory insufficiency, requiring catecholamine administration, endotracheal intubation, and extracorporeal membrane oxygenation. Initially, low-output heart failure due to acute myocarditis was suspected, but the central venous oxygen saturation was high, at 82%. Considering high-output heart failure to be more likely, we evaluated its cause and noted, by urinary organic acid analysis, increases in lactate, pyruvate, 3-OH-butyrate, acetoacetate, metabolic products of branched-chain amino acids, 2-ketoglutarate, 2-OH-glutarate, 2-keto-adipate, and 2-OH-adipate. Since the vitamin B1 level was reduced to 12 ng/mL (normally 20-50 ng/mL), a diagnosis of cardiac beriberi due to vitamin B1 deficiency was made. When unexplained heart failure is observed in children, cardiac beriberi must be excluded as a differential diagnosis of myocarditis and cardiomyopathy. The measurement of the central venous oxygen saturation may be useful for the diagnosis.