A 55-year-old male with a 9-year treatment history of advanced renal cell carcinoma was treated for metastatic mediastinal lymphadenopathy with temsirolimus, a specific inhibitor of the mammalian target of rapamycin (mTOR). A month after the initiation of temsirolimus administration (25 mg/week), the patient started to reveal pyrexia, malaise, and dyspnea. A chest computed tomography taken after an additional month showed interstitial pneumonia. Laboratory tests showed an inflammatory reaction, liver dysfunction, and a high serum level of KL-6. There was lymphocytosis in the bronchoalveolar lavage fluid. This course suggested that temsirolimus caused drug-induced pneumonia, and the patient had been successfully treated with corticosteroids. In Japan, more cases of mTOR inhibitor-induced pneumonia should be collected for the proper and safe usage of temsirolimus.