10-day sequential therapy was not sufficient to overcome tough situation for H. pylori eradication in Korea due to high antimicrobial resistance. The present investigators assumed that doubling duration of second phase of sequential therapy might have more potent bactericidal efficacy than previous 10-day sequential regimen. But 15-day regimen with initial 5-day PPI with amoxicillin followed by remaining 10-day PPI, clarithromycin with metronidazole was not ever tested before. Moreover, whether extending the sequential therapy to 15-day might be more effective than 10-day sequential therapy is unknown especially in Korea. From this background, the present investigators prepared clinical trials regarding modified sequential therapy which was extending the treatment duration to 15 days compared than previous 10-day sequential therapy regimen. In addition, pre-treatment antimicrobial susceptibility testing was performed to find the possibility to overcome antimicrobial resistance.