Myocardial disorder is known as a side effect during chemotherapy with trastuzumab (TRZ) for breast cancer. A 58-year-old woman was diagnosed with carcinoma of the left breast and underwent mastectomy in November 200X-2. TRZ was started as postoperative adjuvant chemotherapy from August 200X-1. In August 200X, left ventricular (LV) wall motion abnormality and ballooning in the apex were detected by echocardiography. Her LV ejection fraction decreased to 38%. After withdrawal of TRZ, the LV wall motion abnormality improved. Occurrence of symptomatic heart failure was reported in 1-4% of patients receiving TRZ, and LV ejection fraction was decreased in 10%. We report a rare case that developed takotsubo cardiomyopathy during TRZ administration.