Disclosed is a pH-sensitive block copolymer that forms polyionic complex micelles. The block copolymer is prepared by copolymerization of (a) a polyethylene glycol compound, (b) a poly(amino acid) compound, and (c) a heterocyclic alkyl amine compound having the ability to induce the formation of ionic complexes. Further disclosed is a drug or protein carrier using the block copolymer.
