PROBLEM TO BE SOLVED: To provide a polymer which has biocompatibility and whose aqueous solution is sol having fluidity which makes it injectable with a syringe and speedily becomes gel； and to provide a crosslinked polymer and injectable hydrogel which are each formed from the polymer； and a kit which is useful for forming the hydrogel.SOLUTION: Injectable hydrogel is obtained by mixing a first aqueous solution with a second aqueous solution which contains a trivalent or higher cation, the first aqueous solution containing a branch-type polymer represented by general formula (a), wherein M represents a hydrogen atom, an alkali metal atom or an alkali earth metal atom of 1/2 atom, m represents an integer of at least 3； [CHCHO]represents a polyoxyethylene group； R represents a linkage group of at least m-valent.
