An 80-year-old woman visited our hospital with a massive ulcerated tumor in the upper lateral quadrant of the right breast. Her performance status was 2. Histopathologically, a mass consisting of a huge primary tumor and metastatic axillary lymph nodes was seen and invasive ductal carcinoma was diagnosed. Both estrogen and progesterone receptors were negative. Herceptest (DakoCytomation, Glostrup, Denmark) showed 2 + staining and HER2 amplification was detected by fluorescent in situ hybridization. CT revealed multiple lung metastases. Her old age and performance status of 2 made aggressive chemotherapy difficult. After receiving 5'-DFUR 600 mg/day as the first line treatment for two months, the tumors progressed. As second-line treatment, single agent therapy with a loading dose, a trastuzumab 4 mg/kg followed by 2 mg/kg weekly was recommended. The patient also received 60 Gy radiotherapy. Six months after the second line treatment, the breast tumor disappeared and only a scar remained on the chest wall and axilla. CT showed no lung tumors. During the trastuzumab treatment, no adverse effect was observed. Her performance status improved to zero, and she is alive and free from the disease 24 months after the disappearance of the tumor.