Abstract: A D-glucose solution is added molybdic acid compound thereto and heated at a temperature of 110.degree.-160.degree. C with pH 2.0-4.5 to perform epimerizing reaction of D-glucose, and thereby 30-36%, based on the D-glucose, of D-mannose is formed. To the epimerized mixture thus obtained is further added glucose-isomerase to perform enzymatic isomerizing reaction of D-glucose remaining in the epimerized mixture, thereby 46% based on the remaining D-glucose is converted into D-fructose. The epimerized mixture or enzymatically isomerized mixture is subjected to catalytic hydrogenation under high pressure, and thereby is produced D-mannitol at high yield based on the initial D-glucose.