The present invention provides a block copolymer applicable as a drug carrier capable of forming a stable complex with a nucleic acid and releasing the nucleic acid in cells. The block copolymer is represented by the following formula (1) or (2)where: R1 and R2 each independently represent a hydrogen atom or a linear or branched alkyl group having 1 to 12 carbon atoms which may have a substituent A represents a hydrophilic polymer chain L1 and L3 each represent a linking group B represents a cation-containing group R3 represents a side chain of any appropriate amino acid z represents an integer of 5 to 500 x represents an integer of 40% or more of z y represents an integer and may represent 0 z-x-y represents an integer and may represent 0 p represents an integer of 1 to 10 and q represents an integer of 1 to 10.
