A carbon-based solid acid which has high activity and high thermal stability and is useful as an acid catalyst for various reactions such as hydration of olefins. The carbon-based solid acid for use as a catalyst is obtained by carbonization and sulfonation of an organic substance, which has a reduction rate of 10 mol % or less of acid content as measured by immersing the solid acid in hot water at 120.degree. C. for 2 hours, is used as the acid catalyst. The organic substance to be used as the raw material for preparing the solid acid is preferably a saccharide having .beta.1-4 glycosidic bond (e.g. cellulose) or lignin. Amylose is also suitable as the raw material. Examples of the reaction for which the solid catalyst can be used include hydration of olefins, etherification of olefins, and acid/alcohol esterification.