A 74-year-old woman was clinically diagnosed with possible amyotrophic lateral sclerosis (ALS) and was administered 100 mg/day of riluzole. After 2 months, she developed dyspnea and experienced gradual difficulty walking. Chest computed tomography revealed ground-glass opacity and consolidation in the lower lobes of both the lungs, thereby suggesting a diagnosis of interstitial pneumonia. Because the condition was suspected to be drug-induced, riluzole administration was discontinued and steroid (methylprednisolone) pulse therapy (1,000 mg/day, 3 days) was started. Her symptoms and radiological findings improved immediately. At 16 months later, she wanted to take riluzole again. She had the similar interstitial pneumonia on the 4(th) day of the re-administration. Drug (riluzole)-induced lymphocyte stimulation tests (DLST) were negative two times. The symptoms of interstitial pneumonia, a rare adverse effect of riluzole, are very similar to worsening symptoms of ALS; therefore, patients with ALS receiving riluzole therapy should be carefully monitored.