An antibacterial agent showing a high affinity for Helicobactor pyroli and having an antibacterial effect specific to H. Pyroli. The agent has a chemical structure wherein an antibacterial substance is bonded to a sulfated polysaccharide or an oligosaccharide prepared by partial decomposition of the sulfated polysaccharide. Preferable embodiments are those having the chemical structure represented by Y--OCH(AH.sub.2NHR).sub.n or Y--BH.sub.2NHR, wherein Y represents a sulfated polysaccharide or an oligosaccharide prepared by partial decomposition of the sulfated polysaccharide; A a carbon derived from aldehyde group occurring through the reduction of the reduced end sugar of Y and subsequent oxidation of the resulting product with an oxidant; B a carbon derived from the aldehyde group at the reduced end sugar of Y; R an antibacterial substance with a primary amino group or with an amino group introduced therein or represents an antibacterial substance derivative prepared by bonding an antibacterial substance through a spacer to the carbon A or the carbon B; and n=1 or 2.