An adhesion preventive material including a cross-linking polysaccharide derivative containing at least one active ester group introduced in a polysaccharide side chain, which is capable of reacting with an active hydrogen-containing group, and being capable of forming a crosslinked material due to covalent binding among the active ester group and an active hydrogen-containing group upon contact with water under an alkaline condition, is disclosed. The adhesion preventive material is able to reduce preparation works to be carried out while previously estimating the time of application and does not require a special device.