An object is to provide a preparation for forming emboli highly safe in a living body and capable of retaining and controlled-releasing an anticancer agent, occluding a blood vessel when injected into the blood vessel, unlikely to be washed out and having a controlled decomposition time (i.e., occludes a blood vessel for a while and quickly decomposes to prevent the necrosis of the entire tissues when the function is completed). The preparation for forming emboli according to the present invention comprises a solution comprising a phenolic hydroxyl group-modified polymer represented by the following formula (1): wherein P is a biocompatible polymer, A is a single bond or an —OCO—C2-C4-alkenylene group, a —CONH—C1-C4-alkylene group or an —HNCO—C1-C4-alkylene group, and X is hydrogen or a C1-C3-alkoxy group, a solution comprising at least one selected from a peroxidase, a laccase, a tyrosinase, a catalase and an iron porphyrin complex and a solution comprising hydrogen peroxide.
