A 66-year-old man was diagnosed with small-cell lung cancer (SCLC; cT1aN2M1a, stage IV, extensive disease) in October 2010. He showed a partial radiological response with first-line chemotherapy (cisplatin and etoposide). The mediastinal lymph node became enlarged in May 2011, and second-line chemotherapy with nogitecan hydrochloride was administered. After 1 cycle of chemotherapy, the size of the lymph node reduced, but the patient experienced rapidly progressive dizziness, opsoclonus, myoclonus, and dysarthria. He was diagnosed with paraneoplastic opsoclonus-myoclonus syndrome (POMS) because of the lack of significant findings after blood examinations, cerebrospinal fluid test, brain magnetic resonance imaging, and electroencephalography. Regardless of steroid therapy, the neurologic symptoms did not improve. In fact, the patient's symptoms worsened, and he exhibited disturbance of consciousness; he died in October 2011. POMS has been reported to occur as a paraneoplastic neurological syndrome in SCLC patients, but it develops before the diagnosis of tumor in most cases. We have reported here a rare case of SCLC associated with POMS that developed during chemotherapy.