To obtain the subject super-absorbing polymer which is biodegradable under aqueous conditions without using a hydrogel intermediate by using polyaziridine and polyepoxide as crosslinking agents.

This crosslinked poly(amino acid) has the stoichiometric formula [A]x[B]y [A is a main chain polymer which is a recurring unit of formula I; B is a crosslinking unit of formula II or formula III which randomly is distributed between units of formula I through formation of ester bond to carboxylate of R group; (x) is wt.% of A based on total weight of A and B; (y) is wt.% of B based on total weight of A and B; (m) is 0, 1 or 2; (n) is 20-20,000; R is C(=O)OM (M is H, an alkali metal, etc.), etc.; X is 0, S or N; R1 is a 1-4C alkylene, aryl, etc.; R1 is a 1-4C alkylene, aryl, etc.; R2 is H or a 1-3C alkyl; R3 is a dipropionic acid ester residue of a 2-6C alkanepolyol; (a) is 0 or 1; (b) is 1-3]. Furthermore, the objective polymer is preferably obtained by bringing an aqueous solution of poly(aminoacid) into contact with a crosslinking agent at pH 3-7 in a reaction mixture.