A hydrogel is provided which is obtained by reacting an azide group and a cyclooctyne group in the absence of a catalyst, especially in the absence of a copper catalyst. The hydrogel has: (a) a first polymer moiety composed of hyaluronic acid, carboxymethyl dextran or the like (b) a second polymer moiety composed of hyaluronic acid, carboxymethyl dextran or the like, said second polymer moiety being of a kind that is same as or different from the kind of the moiety (a) and is composed of a molecule different from the moiety (a) and (c) a triazole ring group or a derivative group thereof.
