Inclusion Criteria:

          -  Provision of informed consent prior to any study specific procedures

          -  Age 20 years or older

          -  ECOG performance status(PS) 0 or 1

          -  Histologically confirmed stage IV or recurrent breast cancer

          -  HER2 negative disease: not eligible for anti-HER2 therapy

             * HER2 negative [IHC 0, 1+ or IHC 2+ with corresponding ISH non-amplified or ratio
             less than 2.0 or ISH non-amplified ratio less than 2.0] as per ASCO-CAP HER2 guideline
             recommendations 2013 (ASCO-CAP)

          -  Patients previously treated with anthracycline and/or taxane unless contraindicated;
             Patients who received anthracycline and/or taxane based chemotherapy in either the
             neoadjuvant, adjuvant or metastatic setting and experienced disease progression on or
             after taxane-based chemotherapy in the metastatic setting

          -  No more than 3 prior lines of cytotoxic chemotherapy for metastatic disease; patients
             who experienced disease recurrence within 1 year after completion of (neo)adjuvant
             anthracycline and taxane-based chemotherapy will be counted as 1 prior line of
             treatment.; hormonal therapy will not be counted as a prior line of treatment

          -  Measurable disease according to RECIST v 1.1.

        Exclusion Criteria:

          -  Previous treatment with eribulin mesylate or any anti-PD-1, PD-L1, or PD-L2

          -  Active autoimmune disease that has required systemic treatment in the past 2 years
             (ie, with use of disease modifying agents, corticosteroids, or immunosuppressive
             drugs). Replacement therapy (eg, thyroxine, insulin, or physiologic corticosteroid
             replacement therapy for adrenal or pituitary insufficiency, etc) is not considered a
             form of systemic treatment.

          -  Known central nervous system (CNS) disease, except for those subjects with treated
             brain metastasis who are stable for at least 1 month, having no evidence of
             progression or hemorrhage after treatment and no ongoing requirement for
             corticosteroids, as ascertained by clinical examination and brain imaging (magnetic
             resonance imaging [MRI] or computed tomography [CT]) during the screening period

          -  Known history of human immunodeficiency virus (HIV) positive

          -  Known active hepatitis B or hepatitis C (eg, HCV RNA detected)

          -  Any other malignancy that required treatment or has shown evidence of recurrence
             (except for nonmelanoma skin cancer, or histologically confirmed complete excision of
             carcinoma in situ) during the 3 years prior to enrollment in this study

          -  History of significant cardiovascular disease

          -  Hypersensitivity to the active substance or any other excipients of the eribulin
             mesylate drug product, or to nivolumab

          -  Scheduled for major surgery during the study

          -  Has a diagnosis of immunodeficiency or is receiving systemic steroid therapy or any
             other form of immunosuppressive therapy within 7 days prior to the first dose of trial
             treatment. The use of physiologic doses of corticosteroids may be allowed

          -  Has a history of (non-infectious) pneumonitis that required steroids or current
             pneumonitis

          -  Has a history of interstitial lung disease

          -  Has received a live-virus vaccination within 30 days of planned start of study
             therapy. Seasonal flu vaccines that do not contain live virus are permitted.