Nivolumab is known to cause immune-related interstitial lung diseases. A 44-year-old woman developed multiple lung metastases 5 years after breast cancer resection. She was treated with docetaxel/trastuzumab/pertuzumab and obtained a complete response. This was a case of off-label use of nivolumab as an immune checkpoint inhibitor evaluated in a private clinic. After standard therapy with T-DM1, she developed fever and hypoxemia. Drug-induced pneumonia was suspected based on a ground-glass shadow finding in chest computed tomography. Drug-induced pneumonia may develop following molecular-targeted therapy delivered after nivolumab administration, and fatal cases have been reported. The antitumor effects of nivolumab for breast cancer have not been proven, and its off-label use may have possible adverse effects on future treatments.