An 83‒year‒old woman received trastuzumab plus anastrozole as first‒line chemotherapy for inflammatory breast cancer in her left breast. Following the treatment, the induration and redness in her breast gradually improved; however, 2 days after receiving the 5th course of chemotherapy, she developed dyspnea and was referred to the emergency room. Her SpO2 was 88%; her KL‒6 level had increased to 2,613 U/mL; and a chest CT scan showed ground‒glass opacity in the bilateral lung fields, yielding a diagnosis of interstitial pneumonia requiring steroid pulse therapy. The dyspnea improved immediately after steroid administration, and the patient was discharged 20 days after hospitalization. Thereafter, the steroid dosage was gradually lowered to 5 mg/day. We discontinued steroid therapy after a chest CT confirmed the reduction of ground‒glass opacity. However, she was later readmitted for interstitial pneumonia for which she was readministered steroid pulse therapy. Trastuzumab‒induced interstitial pneumonia is rare, but we must be aware of the possibility that patients may develop severe pulmonary disorders or experience cardiotoxic effects.