Inclusion Criteria:

          1. Over 19 years old

          2. Histologically confirmed HER2 overexpressing gastric cancer (HER2 overexpression is
             defined as IHC 3+ or FISH +)

          3. Metastatic gastric cancer

          4. Progressive disease has been confirmed after first line treatment including
             trastuzumab (If the recurrence occurred within 6 months after the completion of
             postoperative adjuvant therapy, it can be considered as first line treatment.)

          5. At least one measurable or evaluable lesion according to RECIST ver 1.1

          6. ECOG performance status 0 or 1

          7. Appropriate major organ function as defined below, A. Absolute neutrophil count (ANC)
             ≥ 1,500/mm3 B. Platelet ≥ 100,000/mm3 C. Hemoglobin > 8.0 g/dL D. Total bilirubin ≤
             1.5 x ULN E. AST and ALT < 3 x ULN (if there is a live metastasis, AST and ALT ≤ 5 x
             ULN) F. Serum creatinine ≤ 1.5x ULN or CCr > 50 mL/min

          8. Life expectancy is more than 12 weeks

          9. Echocardiography at the time of enrollment showed an ejection fraction ≥ 50%

         10. Previous adverse events of chemotherapy or radiotherapy has been resolved to less than
             grade 1 toxicity (exception: Alopecia, stable peripheral neuropathy, and manageable
             electrolyte imbalance by replacement therapy are acceptable if they are resolved to
             less than grade 2)

         11. If the urine pregnancy test or serum beta-hCG result is negative in child bearing
             women

         12. If the subject have signed the informed consent form approved by the IRB

        Exclusion Criteria:

          1. Symptomatic or unstable CNS metastasis (exception: appropriately treated brain
             metastasis without progression of more than 4 weeks after previous CT or MRI, and no
             steroid treatment for symptom relief is necessary)

          2. Active virus, bacteria, or fungal infection (exception: HBV DNA titer is in the normal
             range for chronic hepatitis B carriers)

          3. If previous chemotherapy or radiotherapy was applied within 3 weeks before the
             administration of study drug

          4. If the subject has received major surgery within 4 weeks and the recovery is not
             complete before the administration of study drug

          5. If there is a history of other malignancies within 3 years (exception: cervical
             intraepithelial cancer, well differentiated thyroid cancer, or skin cancer has been
             treated completely)

          6. QTc interval > 480 msec, long or short QT syndrome, or Burgada syndrome. In addition,
             known prolongation of QTc interval or Torsade de Pointes

          7. If there is a significant history of cardiovascular disease within 6 months, such as,
             myocardial infarction, unstable angina, significant cardiac arrhythmia, and
             uncontrolled hypertension (systolic BP > 180 mmHg, diastolic BP > 100 mmHg),
             congestive heart failure (NYHA class III-IV), pericardial effusion or pericardial
             tamponade, cardiomyopathy, cerebrovascular disease including transient ischemic
             stroke, and symptomatic pulmonary embolism.

          8. History of symptomatic interstitial pneumonia

          9. History of other psychiatric problems, abnormalities of laboratory test which have
             potential effects on administration of study drugs or participation on the study, or
             if the subject is inappropriate to be participated according to the investigator's
             decision (refusal to request and instruction, incooperative attitudes, etc.)