Disclosed are: a novel polyethylene glycol derivative； a drug carrier comprising the derivative； and a pharmaceutical composition comprising the drug carrier having a pharmacologically active substance included therein. Specifically disclosed are: a polyethylene glycol derivative represented by the general formula (I)； a drug carrier comprising the derivative and 2-0-(2-diethylaminoethyl)carbamoyl- 1,3-O-dioleoyl-glycerol； and a pharmaceutical composition comprising the drug carrier and a pharmacologically active substance (preferably double-stranded RNA, double-stranded DNA or an oligonucleic acid). Wherein R represents a saturated or unsaturated aliphatic hydrocarbon group having 10 to 30 carbon atoms or a saturated or unsaturated fatty acid residue having 10 to 30 carbon atoms； and n represents an integer of 30 to 150.
