A 77-year-old man was admitted to our hospital with muscle weakness and shortness of breath. Chest CT showed a mass in the left upper lobe, and electromyography showed waxing phenomenon with high-frequency repetitive stimulation. We diagnosed Lambert-Eaton myasthenic syndrome accompanying small cell lung cancer. He was treated with carboplatin and etoposide, and concurrent thoracic irradiation. Although, the tumor size decreased, his myasthenic symptoms remained. He started taking 3,4-diaminopyridine and his muscle weakness improved dramatically, and he was eventually able to walk finally. In some cases, anti-tumor therapies cannot improve the myasthenic symptoms. In such cases, 3,4-diaminopyridine could improve the quality of life, and should be approved in Japan.