Disclosed is a pH-sensitive block copolymer obtained by copolymerization of: (a) a polyethylene glycol compound (A) and (b) at least one poly(amino acid) compound selected from the group consisting of a poly(β-amino ester) and poly(amido amine) or a copolymer thereof (B). A method for preparing the same block copolymer, and a polymer micelle type drug composition comprising the pH-sensitive block copolymer and a physiologically active substance that can be encapsulated with the block copolymer are also disclosed. The pH-sensitive block copolymer is obtained by polymerization of a hydrophilic polyethylene glycol compound with a pH-sensitive biodegradable poly(amino acid) compound. Therefore, the pH-sensitive block copolymer can form a micelle structure due to its amphiphilicity and ionization characteristics depending on pH variations, and thus can be used as drug carrier for target-directed drug delivery depending on pH variations in the body.
