An object of the present invention is to provide an anti-adhesion membrane that has no toxicity to a living body, has flexibility allowing itself to fit an affected part as a hydrated gel, is uniformly crosslinked, and is immediately absorbed in a living body after maintaining its shape in the living body for a certain period of time. The present invention provides anti-adhesion material, which comprises a thermally crosslinked gelatin film, and has a water content of 60 to 85% calculated by the following formula (1): water content (%)=[(Ws-Wd)/Ws].times.100(%) (1), in the formula (1), Ws representing a weight (wet weight) of the anti-adhesion material immersed in a phosphate buffered saline solution at a temperature of 25.degree. C. for one hour, and Wd representing a weight (dry weight) of the anti-adhesion material dried completely using a vacuum drying apparatus.