[Problem] To provide a drug delivery system which is retained in the blood circulation and improves pharmacological effect of the drug in the target organ and cell. [Means for Resolution] A drug-containing liposome for use in the treatment with a liposome prepared by employing polyethylene glycol cholesteryl ether as a part of the liposome-forming agent and using said liposome-forming agent. It is possible to obtain excellent drug effect by further employing a polyethylene glycol diacylglycerol ester as a part of the liposome-forming agent.
