Abstract: The resin solid acid is a sulfo group-modified resin obtained by introducing sulfo groups into a raw material resin in an uncarbonized state, the yield of the sulfo group-modified resin based on the weight of the uncarbonized raw material resin is 80% or more, the amount of sulfo groups in the sulfo group-modified resin is 1 mmol/g or more, and the raw material resin is in the form of a powder, granules or fibers. In addition, the method for producing the resin solid acid is a production method for obtaining a sulfo group-modified resin by comprising a step for adding a sulfonating agent in the form of any of sulfuric acid, fuming sulfuric acid or chlorosulfonic acid to a raw material resin in an uncarbonized state, and a step for heating the uncarbonized raw material resin at 200° C. or lower.