Provided is a production method that permits industrial mass production of modified xylopolysaccharides from a biomass. A modified xylopolysaccharide that conserves the modifying group from the side chain of the xylan is obtained by using as the raw material a biomass containing xylan in plant cell walls, the xylan having at least one modifying group selected from the group comprising an acetyl group, feruloyl arabinofuranosyl group, and coumaroyl arabinofuranosyl group in a side chain, and conducting continuous hot-water treatment of a slurry including the biomass in a ratio of 10-30 mass% as the solid content under controlled conditions consisting of a temperature of 160°C or higher, a pressure of the saturated water vapor pressure at this temperature or above, and a reaction severity R 0 calculated by formula (1) of 3000-7000 using a cylindrical plug flow reactor equipped internally with a flow path control mechanism for generating a plug flow.