The present invention relates to an antimicrobial agent for preventing, ameliorating and treating diseases attributable to Helicobacter pylori, for example peptic ulcer and gastritis, particularly recurring peptic ulcer and recurring gastritis, and provides (1) an antimicrobial agent comprising oxethazaine represented by the following formula as an active ingredient, (2) an antimicrobial agent comprising oxethazaine and a proton pump inhibitor as active ingredients, and (3) an antimicrobial agent comprising oxethazaine and an antibiotic as active ingredients.