Nivolumab is an immune checkpoint inhibitor with demonstrated efficacy against several malignant tumors. Alterations in driver oncogenes such as EGFR and ALK are a poor prognostic factor in nivolumab therapy for non-small cell lung cancer (NSCLC), whereas a smoking history is a well-known, favorable prognostic factor. However, an efficacy of nivolumab therapy for multiple primary malignant tumors (MPMTs) has not been reported, and its efficacy for driver oncogene-positive NSCLC in smokers is unclear. Herein, we report the case of a patient with a history of heavy smoking who developed synchronous ALK-positive NSCLC and gastric cancer that responded to nivolumab therapy. A 76-year-old man who was a heavy smoker presented to our hospital with symptoms of hoarseness and dysphagia. He was ultimately diagnosed with ALK-positive advanced NSCLC. An ALK inhibitor (alectinib) was administered, and the lung cancer lesions showed improvement. The alectinib therapy was continued for 5 months. Thereafter, the lesions in the left lower lobe of the lung showed regrowth. During the same period, the patient experienced epigastric pain. Gastrointestinal endoscopy examination revealed gastric cancer. He was administered nivolumab to treat both the lung cancer and the gastric cancer. Two months later, both the lung lesions and the gastric lesions had diminished in size. Nivolumab therapy might be an effective therapy for synchronous MPMTs and NSCLC in heavy smokers, even if the lung cancer possesses driver oncogene mutations.