Although nivolumab was previously reported to cause immune-related interstitial lung diseases (ILD), the detailed characteristics of ILD in gastric cancer are not fully understood. We herein present a rare case of a 66-year-old male with advanced gastric cancer who experienced acute-onset high-grade fever and dyspnea and diagnosed with early-onset ILD during the first cycle of nivolumab. Computed tomography revealed patchy infiltrative shadows and ground-glass opacities. No pathological bacteria were detected in the sputum or the bronchoalveolar lavage, and serous antigens for virus and beta-D-glucan were below the detection limit. These findings were consistent with nivolumab-induced organizing pneumonia. The steroid pulse therapy was effective for ILD, and the patient had complete radiological response, although he relapsed twice during the steroid tapering period.