Switching From Tenofovir Disoproxil Fumarate to Besifovir Dipivoxil Maleate

Inclusion Criteria: 1. 20 years of age and older, Male or female patients 2. Patients who show positive HBsAg or has a history of chronic hepatitis B for the last six months or more before screening 3. Patients who have been on tenofovir disoproxil fumarate (TDF) monotherapy for more than 48 weeks and are taking TDF at the time of clinical screening 4. At screening, had HBV DNA \< 20 IU/mL 5. Patients who were explained about the purpose, methods and effects of the clinical trial and then, signed a written consent form Exclusion Criteria: 1. Patients who have received interferon (including Pegylation formulation) to treat chronic hepatitis for more than 12 months. 2. Patients who have taken Besifovir 3. Patients who have experienced hepatitis B virus resistance to antiviral drugs 4. Patient diagnosed with a malignant tumor within 5 years before screening or relapsed patient 5. Patient has history of organ transplantation 6. Patients who had received the following drugs for the last two months before screening (however, short-term use (less than consecutive 14 days) of these drugs and low-dose aspirin (100 mg, maximally, 300 mg/day) are allowed.) * Nephrotoxic drugs (e.g. Aminoglycosides, Amphotericin B, NSAIDs) * Hepatotoxic drugs (e.g. Erythromycin, Ketoconazole, Rifampin, Fluconazole, Dapsone) * Anticoagulant (e.g. Warfarin) 7. Patients who are suspected by an investigator to have the level of immunity decreased among patients who had been administered with immunosuppressants within 12 months before screening 8. Patients who had been administered with long-term general corticosteroids (more than consecutive 14 days) at a high dose (more than prednisolone 20 mg daily\*) within three months before screening (In case of local corticosteroids, an investigator decides it.) * It is equal to cortisone 125 mg, hydrocortisone 100 mg, prednisone 20 mg, methylprednisolone 16 mg, triamcinolone 16 mg, dexamethasone 3 mg, betamethasone 2.4 mg 9. Patients who have a past medical history of clinical alcohol or drug abuse within a year before screening or now are abusers 10. Patients with hepatitis C virus, hepatitis D virus or human immunodeficiency virus 11. Patients who have other hepatic diseases (hematochromatosis, Wilson's disease, alcoholic liver diseases, nonalcoholic steatohepatitis, α1-antitrypsin deficiency) except hepatitis B 12. Patient concerned about the decline in daily activity or not able to understand the objectives and methods due to the psychiatric problems 13. Patients who showed Glomerular Filtration Rate (GFR) less than 50 mL/min by calculating Modification of Diet in Renal Disease (MDRD: 1.86 x phosphocreatine -1.154 x age -0.203 (x 0.742 for women)) during screening 14. Patients who showed alpha-fetoprotein(AFP) more than 50 ng/mL during screening and are estimated to have hepatocellular carcinoma (HCC) through liver/abdomen CT scans 15. At least one of the following laboratory values during screening * Hemoglobin \< 9.0 g/dL * Absolute neutrophil count (ANC) \< 1.0 x 10\^9 /L (1000 /mm\^3) * Platelet count \< 75 x 10\^9 /L (100 x 10\^3 /mm3) * Serum creatinine \> 1.5 mg/dL * Serum amylase \> 2 x upper limit normal (ULN) and Lipase \> 2 x ULN * Total Bilirubin \> 2 x ULN * Serum albumin \< 28 g/L (2.8 g/dL) 16. Pregnant women, lactating women, or patients who planned pregnancy during a trial period 17. Patients who participate in other clinical trials or is supposed to do so during the study period 18. Patients who have hypersensitivity to the clinical trial drug in this clinical trial 19. Patients with genetic problems such as galactose intolerance, Lapp lactase deficiency or glucose-galactose malabsorption 20. Patients who are considered to be unacceptable in this study under the opinion of the investigator