Proposed is a method for vascular embolization of the blood vessel of a patient by introducing an aqueous solution of a specific thermosensitive polymer, which is liquid at low temperatures but causes coagulation when heated up to the body temperature of the patient, into the blood vessel followed by in situ heating of the solution. The thermosensitive polymer found to be suitable for the purpose is a homopolymer or copolymer of an N-substituted (meth) acrylamide monomer having a specified intrinsic viscosity in tetrahydrofuran and gives an aqueous solution capable of exhibiting phase transition from a liquid to a coagulate at a transition temperature of 10.degree. to 37.degree..