An object of the present invention is to prepare substances which are excellent in delivery and enable drugs to be retained in a body effectively over a long period and to construct a drug delivery system using the substances. When the delivering substance which is obtained by reacting polyalkylene glycol or a reactive derivative thereof, a phospholipid and a drug with each other to form covalent bonds is administered systemically or topically, the substance is retained at a target site in a body for a long period, thereby making it possible to sustain drug efficacy over a long period by a single administration.