We report that single agent therapy with trastuzumab had a significant effect on metastatic breast cancer, which was confirmed to be HER2 positive by Herceptest showing 2+staining, and gene amplification positively detected by FISH analysis. A 48-year-old woman underwent extended radical mastectomy (T2N0M0 stage II). Three years after the operation supraclavicular lymph node metastasis was noted. Bone scintigraphy showed metastases to the left ribs 5 years after operation. She was treated with chemo-endocrine therapy, but nonetheless could not bear the back pain caused by the bone metastases. Another chemotherapy course could not be permitted because of leukopenia. Immunohistochemistry (IHC) analysis with Herceptest showed 2+staining for HER2 and FISH analysis showed gene amplification of HER2. We started single agent therapy with trastuzumab and she subsequently had remarkably improved back pain. Physical examination and ultrasonography showed disappearance of the previous palpable supraclaviclar lymph nodes. Serum tumor markers were also reduced after the first administration of trastuzumab. The patient is currently alive, with no further progression of the lymph node or bone metastases.