Thiazolium ion catalyst was immobilized on a cation-exchange resin, and the formose reaction was carried out with the resin. As a result, the use of this resin turned out to be useful for the formose reaction in an organic solvent, similar to the free thiazolium catalyst. Interestingly, the catalytic activity of this resin decreases after washing with an organic solvent, but recovers after washing with water. This result suggests that water molecules have an influence on the catalysis in this system.