Inclusion Criteria:

          1. Stage IIIB or IV lung cancer diagnosed radiologically with or without pathologic
             diagnosis

          2. Age> 18 year-old

          3. ECOG performance status 0~2.

          4. Activating EGFR mutation (G719X, exon 19 deletion, L858R, L861Q) detected from
             circulating DNA

          5. Any one of the following criteria should be met

               -  Unavailable or failed pathologic/cytologic diagnosis

               -  Wild type or failed EGFR testing based on tumor tissue

               -  No more tumor sample available for EGFR test

          6. Measurable lesion by RECIST v1.1

          7. Females should be using adequate contraceptive measures, should not be breast feeding
             and must have a negative pregnancy test prior to start of dosing or evidence of
             non-child bearing potential.

          8. Male patients should be willing to use barrier contraception.

          9. Provision of signed and dated, written informed consent prior to any study specific
             procedures, sampling and analyses

         10. Adequate organ function, defined as all of the following:

               -  Absolute neutrophil count (ANC) >=1500/mm3

               -  Platelet count >= 75,000 /mm3

               -  Serum creatinine < 1.4 mg/dL

               -  AST or ALT < three times the upper limit of normal

        Exclusion Criteria:

          1. Prior exposure to EGFR-TKI. Prior chemotherapy will be permitted.

          2. Previous or concomitant malignancies at other sites, except effectively treated
             non-melanoma skin cancers, carcinoma in situ of the cervix, ductal carcinoma in situ
             or effectively treated malignancy that has been in remission for more than 3 years and
             is considered to be cured.

          3. Severe or unstable medical conditions such as history or presence of clinically
             relevant cardiovascular abnormalities such as uncontrolled hypertension, congestive
             heart failure NYHA classification of ≥ 3, unstable angina or poorly controlled
             arrhythmia as determined by the investigator. Myocardial infarction within 6 months
             prior to randomisation.

          4. Known pre-existing interstitial lung disease

          5. Any history or presence of poorly controlled gastrointestinal disorders that could
             affect the absorption of the study drug (e.g. Crohn's disease, ulcerative colitis,
             chronic diarrhoea, malabsorption)

          6. Active hepatitis B infection (defined as presence of HepB sAg and Hep B DNA), active
             hepatitis C infection (defined as presence of Hep C RNA) and/or known HIV carrier.