A 54-year-old man was diagnosed as syndrome of inappropriate secretion of antidiuretic hormone (SIADH) 7 days after lung transplantation, whereas the preoperative serum sodium level was normal. Hypertonic saline infusion with furosemide did not improve hyponatremia, however, tolvaptan corrected his serum sodium levels from 123 mEq/L to 131 mEq/L. Seven days after maintenance of tolvaptan, this drug was discontinued and hyponatremia did not occur. Herein, we report a case of SIADH after lung transplantation treated with tolvaptan.