To develop a reversible forming polymer network system, a polymer complexation was conducted between water-soluble and cytocompatible polymers, namely poly(2-methacryloyloxyethyl phosphorylcholine-co-n-butyl methacrylate-co-p-vinylphenylboronic acid) (PMBV) and poly(vinyl alcohol) (PVA). The two polymers spontaneously form hydrogel upon mixing. The association constant depended on the chemical structure of the polyol compounds and the aggregation of the PMBV. Compared to the association constants of PVA to PMBV, D-glucose and D-sorbitol have smaller and larger association constants, respectively, which is in good accord with the dissociation phenomenon of PMBV/PVA hydrogels. In conclusion, consideration of the association constants between the PMBV and polyol compounds is important for obtaining good reversibility of a polymer hydrogel system.