A Study of Eribulin Mesylate With Trastuzumab for Advanced or Recurrent Human Epidermal Growth Factor Receptor 2-Positive (HER2+) Breast Cancer

Inclusion Criteria * Females aged greater than or equal to 20 years and less than 75 years at the time of informed consent. * Histologically or cytologically confirmed with breast cancer * Score 3+ by immunohistochemistry (IHC) or HER2 positive by Fluorescence in Situ Hybridization (FISH) method * Subjects who meet any of the following criteria: * Evidence of recurrence during adjuvant chemotherapy with trastuzumab and taxane * Evidence of recurrence within 6 months after adjuvant chemotherapy with trastuzumab and taxane * Experienced prior chemotherapy including trastuzumab and taxane for advanced or recurrent breast cancer * Adequate organ function * Eastern Cooperative Oncology Group (ECOG)-Performance Status (PS) is 0 or 1 * Subjects who have submitted written informed consent for study entry Exclusion Criteria * Subjects with known brain metastasis accompanied by clinical symptoms or requiring active treatment * Subjects with severe active infection requiring active treatment * Subjects with large pleural effusions, ascites, or pericardial effusions requiring drainage. * Hypersensitivity to trastuzumab, halicondrin B or halicondrin B chemical derivatives * Known positive for human immunodeficiency virus (HIV) test or positive for hepatitis B surface (HBs antigen) or hepatitis C (HCV) by serum test. * Subjects who are pregnant (positive B-hCG test) or breastfeeding * Subjects judged to be ineligible for this study by the principal investigator or sub-investigator.