The characteristic adverse events of olaparib, a PARP inhibitor, are nausea, vomiting, and anemia, and interstitial pneumonia is rarely reported. We report a case of interstitial pneumonia following the treatment of a metastatic breast cancer with olaparib. The patient was a 34-year-old woman. In March 2018, she was diagnosed with stage Ⅳ breast cancer(multiple lung metastases). She was treated with epirubicin and cyclophosphamide followed by paclitaxel. In November 2018, brain and spinal cord metastases were detected, and she was treated with radiation. In December 2018, a BRCA1 deleterious mutation was confirmed, and treatment with olaparib was initiated. Six weeks later, olaparib was discontinued due to anemia; it also caused interstitial pneumonia. The interstitial pneumonia resolved following multidisciplinary treatment during hospitalization. Subsequently, she was treated with cyclophosphamide/methotrexate/fluorouracil. It is necessary to consider interstitial pneumonia as an adverse effect of olaparib.