A retention material for a gas processing device including a processing structure and a casing for accommodating the processing structure, the retention material including inorganic fibers and being arranged between the processing structure and the casing, wherein in a test of repeating a cycle of compressing the retention material until a bulk density of the retention material becomes a prescribed compression bulk density, followed by retaining for 10 seconds, and then releasing until a bulk density of the retention material becomes a release bulk density that is smaller by 12% of said prescribed compression bulk density; a release surface pressure of the retention material after repeating the cycle 2500 times and the compression bulk density of the retention material satisfies the relationship, P≥17.10×D−1.62 wherein P is the release surface pressure (N/cm2) and D is the compression bulk density (g/cm3).