About 600 patients who failed from first line triple therapy or sequential therapy will be eligible in this multicenter randomized comparative trial. Eligible patients will be randomized to receive either (1) levofloxacin-based sequential therapy for 10 days〔lansoprazole 30mg and amoxicillin 1g for the first 5 days, followed by lansoprazole 30mg, levofloxacin 250mg, and metronidazole 500mg for another 5 days (all given twice daily)〕or (2) levofloxacin-based triple therapy for 10 days〔lansoprazole 30mg and amoxicillin 1g, and levofloxacin 250mg for 10 days (all given twice daily)〕. Eradication will be confirmed with 13C-Urea Breath Test 6 weeks after therapy.