To obtain the subject inhibitor having excellent antibacterial activity against Helicobacter pylori, also high in safety by using, as active ingredient, a specific compound afforded by soybean or processed products thereof.

This Helicobacter pylori inhibitor contains, as active ingredient, an isoflavone or saponin. The isoflavone may be either of glycoside type or hydrolyzed aglycone portion, for example, daidzin, daidzein, genistin; while the saponin may be either of glycoside type or sapogenin as an aglycone. For the isoflavone or saponin, a combination of one or more kinds thereof can be used. Because these active ingredients are rich in soybean or processed products thereof, these foods can be used directly. The daily effective amount of this inhibitor is 0.01-100 mg per adult in terms of the isoflavone or saponin.