This invention provides an embolization material used for blocking a blood vessel in vivo for stopping the blood flow. The most suitable embolization material has a water swelling ratio of 30% or more, is degradable in a phosphate buffered saline, is formed as virtually spherical particles, and is preferably composed of a water insoluble poly(ethylene glycol) copolymer, wherein when the film formed from said polymer is saturated with water, it has an elastic modulus in tension of 1500 MPa or less.The embolization material of this invention can reliably block a blood vessel at an intended site without causing cohesion or clogging in a catheter or in the blood vessel at other than the intended site. Thereafter, the blocked site concerned can be liberated from the embolized state by degradation, and the degraded components can be metabolized or excreted outside the body.
