A Multicenter Trial to Investigate the Efficacy and Safety of Tolvaptan in Patients With Hyponatremia in SIADH

Inclusion Criteria: * Subjects with a definite diagnosis of SIADH in reference to "Diagnostic and Treatment Manual of the Hypersecretion of Vasopressin (SIADH), Revised in 2011" * Subjects who have been on fluid restriction for at least 7 consecutive days at the time of informed consent and who are showing no improvement of hyponatremia at the time of the screening examination Exclusion Criteria: * Subjects who have transient hyponatremia induced by drug administration * Subjects who are unable to sense thirst or who have difficulty with fluid intake * Subjects with urinary tract obstruction * Subjects who have participated in any other clinical trial within 30 days prior to informed consent * Subjects with serum sodium concentration of \<120 mEq/L associated with neurologic impairment, including apathy, confusion, or seizures