A 71-year-old woman was diagnosed with EGFR mutation-positive adenocarcinoma (cT2aN2M1b, stage IV) of the lung with multiple bone metastases and carcinomatous pleuritis. The first-line treatment gefitinib (250 mg daily) was administered until hepatopathy occurred. The second-line treatment, erlotinib (100 mg daily), was begun but discontinued because pleural fluid increased and serum carcinoembryonic antigen (CEA) rose. Next she received the third-line treatment combining carboplatin and pemetrexed. Six days after the day of administration of the second cycle of chemotherapy, she was diagnosed as syndrome of inappropriate secretion of antidiuretic hormone (SIADH) induced by carboplatin. Multiple brain metastases were then pointed out by magnetic resonance imaging (MRI), but they were unrelated to SIADH. SIADH induced by carboplatin is often learned about, but there are few reports written to explain what is learned. It is necessary to consider the possibility that carboplatin could cause SIADH after the second cycle of administration.