Abstract: By administering a liposome formulation in which an antibacterial agent is bound to a liposome, in particular, a lipid-soluble side chain of the antibacterial agent is bound to a lipid of the liposome, and the antibacterial agent extends outward from a surface of the liposome, it is possible to provide a liposome formulation in which blood retention of an active ingredient is increased, an amount of the active ingredient taken by a reticuloendothelial system such as a liver is reduced, an amount of the active ingredient transferred into a kidney is reduced, and antibacterial activity can also be increased with little resistance.