The object of the present invention is to provide an embolizing material which is easily handleable and stable in quality and is solved by providing an embolizing material comprising an ester derivative of a polysaccharide having a weight-average molecular weight of 10,000-500,000 daltons and an average substitution degree of acyl group of over two; an embolizing agent for arteriovenous aneurysm, which comprises the embolizing material and an appropriate physiologically acceptable water-soluble organic acid; and a method of producing an embolizing agent, which comprises a step of dissolving the embolizing material in an appropriate physiologically acceptable water-soluble organic solvent.