The present invention provides a cationic polymer which is much closer to the component of a human body, and capable of forming a polyion complex together with an anionic material such as hyaluronic acid. The cationized hyaluronic acid of the present invention is a hyaluronic acid wherein at least one part of hydroxylic hydrogen atoms of hyaluronic acid is replaced with a group having a quaternary ammonium cation group. The cationized hyaluronic acid can be synthesized by reacting hyaluronic acid with a cationizing agent such as glycidyltrialkyleammonium halide. The cationized hyaluronic acid of the present invention can form a polyion complex gel with hyaluronic acid or the like in the presence of water.