We report an elderly male patient with hyperammonemia induced by intrahepatic portal-systemic shunt without cirrhosis (IPSSwoC). The occasional emergence of his erratic behaviors was misdiagnosed as a psychiatric disorder. Regardless of his uneven symptoms, IPSSwoC was suspected due to his hyperammonemia. The contrast computed tomography of the abdomen revealed a congenital type of IPSSwoC. As blood ammonia levels are inconstant, repeated blood tests are recommended when this disease is suspected in elderly patients with psychiatric symptoms.