Adjuvant Pembrolizumab in N2 Positive Non-small Cell Lung Cancer Patients

Inclusion Criteria: * In order to be eligible for participation in this trial, the subject must: 1. Be willing and able to provide written informed consent for the trial 2. Be 18 years of age 3. Be within 6 weeks after complete resection after neoadjuvant CCRT 4. Have a performance status of 0 or 1 on the ECOG Performance Scale. 5. Patients who completed neoadjuvant CCRT regimen includes 5 cycles of weekly Paclitaxel (50 mg/m2 + 5DW200 MIV over 1hr) plus Cisplatin (25 mg/m2 + N/S 150 mL MIV over 1hr) concurrent with radiotherapy (44 Gy/22fx daily fraction). 6. Be willing to provide tissue from a obtained before neoadjuvant CCRT and surgical specimen (at least 10 slides from each surgically resected primary tumors and surgically resected metastatic lymph nodes) 7. Demonstrate adequate organ function as defined in Table 1, all screening labs should be performed within 14 days of treatment initiation. 8. Female subject of childbearing potential should have a negative urine or serum pregnancy within 72 hours prior to receiving the first dose of study medication. If the urine test is positive or cannot be confirmed as negative, a serum pregnancy test will be required. 9. Female subjects of childbearing potential (Section 5.7.2) must be willing to use an adequate method of contraception as outlined in Section 5.7.2 - Contraception, for the course of the study through 120 days after the last dose of study medication. Note: Abstinence is acceptable if this is the usual lifestyle and preferred contraception for the subject. 10. Male subjects of childbearing potential (Section 5.7.1) must agree to use an adequate method of contraception as outlined in Section 5.7.1- Contraception, starting with the first dose of study therapy through 120 days after the last dose of study therapy. Note: Abstinence is acceptable if this is the usual lifestyle and preferred contraception for the subject. Exclusion Criteria: * The subject must be excluded from participating in the trial if the subject: 1. Is currently participating and receiving study therapy or has participated in a study of an investigational agent and received study therapy or used an investigational device within 4 weeks of the first dose of treatment. 2. Participation in another clinical study with PD-1 or PD-L1 inhibitors at any time 3. Any previous treatment with a PD-1, PD-L1 inhibitor or any anti-cancer therapy after complete resection of lung cancer 4. Current or prior use of immunosuppressive medication within 28 days before the first dose of pembrolizumab, with the exceptions of intranasal and inhaled corticosteroids or systemic corticosteroids at physiological doses, which are not to exceed 10 mg/day of prednisone, or an equivalent corticosteroid 5. Mean QT interval corrected for heart rate (QTc) ≥470 ms using Bazett's correction 6. Has a known history of active TB (Bacillus Tuberculosis) 7. Active or prior documented inflammatory bowel disease (Crohn's disease or ulcerative colitis) 8. History of allogenic organ transplant 9. Hypersensitivity to pembrolizumab or any of its excipients. 10. Prior history of malignancy within 2 years from study entry except for adequately treated basal cell or squamous cell skin cancer or in situ cervical cancer, or early gastric cancer. The incidentally detected, non-metastatic well differentiated thyroid cancer can be enrolled irrespective of the treatment, because the prognosis of this type of cancer is known much better than the study disease, even it is untreated. For the debatable double primary cancer, the enrolment can be discussed with the principal investigator in a manner of case by case, and if it is not expected to affect the study outcome, the subjects can be enrolled. 11. Has active autoimmune disease that has required systemic treatment in the past 2 years (i.e. 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. 12. Has known history of, or any evidence of active, non-infectious pneumonitis. 13. Evidence of interstitial lung disease. 14. Has an active infection requiring systemic therapy. 15. Has a history or current evidence of any condition, therapy, or laboratory abnormality that might confound the results of the trial, interfere with the subject's participation for the full duration of the trial, or is not in the best interest of the subject to participate, in the opinion of the treating investigator. 16. Has known psychiatric or substance abuse disorders that would interfere with cooperation with the requirements of the trial. 17. Is pregnant or breastfeeding, or expecting to conceive or father children within the projected duration of the trial, starting with the pre-screening or screening visit through 120 days after the last dose of trial treatment. 18. Has a known history of Human Immunodeficiency Virus (HIV) (HIV 1/2 antibodies). 19. Has known active Hepatitis B (e.g., HBsAg reactive) or Hepatitis C (e.g., HCV RNA \[qualitative\] is detected). 20. Has received a live vaccine within 30 days of planned start of study therapy. Note: Seasonal influenza vaccines for injection are generally inactivated flu vaccines and are allowed; however intranasal influenza vaccines (e.g., Flu-Mist®) are live attenuated vaccines, and are not allowed