A method for producing a block copolymer represented by formula: wherein: A represents hydrogen or an unsubstituted or substituted C 1 -C 12 alkyl group, wherein a substituent when the group is substituted represents a formyl group, a group of the formula R 1 R 2 CH- (where R 1 and R 2 independently represent C 1 -C 4 alkoxy or R 1 and R 2 together represent -OCH 2 CH 2 O-, -O(CH 2) 3 O- or -O(CH 2) 4 O-); L represents -O-(CH 2) a -NH-, -O-(CH 2) a -O-, -(CH 2) a -NH- or -(CH 2) a -O-, wherein a is an integer of 1 to 6 and the orientation of the linkage group L is the forward direction of each moiety in each structural formula; Y represents H, C 1-21 alkylcarbonyl, substituted C 1-4 alkylcarbonyl, unsubstituted or substituted C 3-7 cycloalkylcarbonyl, unsubstituted or substituted arylcarbonyl, or unsubstituted or substituted 5- or 6-membered heteroarylcarbonyl, wherein a substituent of the substituted C 1-4 alkylcarbonyl is selected from the group consisting of a halogen atom, hydroxyl, carboxyl, unsubstituted or substituted C 3-7 cycloalkyl, unsubstituted or substituted aryl, unsubstituted or substituted 5- or 6-membered heteroaryl, unsubstituted or substituted adamantyl and an unsubstituted or substituted cholesterol residue, wherein a substituent when the foregoing substituents are substituted may be C 1-4 alkyl, C 1-4 alkyloxy, hydroxyl, carboxyl, cyano, nitro, a halogen atom or mono- or di-C 1-4 alkylamino; m is an integer of 5 to 300; n is an integer of 5 to 1,000; and up to 80% of m amidino groups (C(=NH)NH 2) may be replaced by H, the method comprising a step of reacting a block copolymer represented by formula: wherein: A, L, Y, n and m are as defined for formula above, with N,N'-bis(tert-butoxycarbonyl)-1H-pyrazole-1-carboxamidine optionally in an inert solvent in order to convert a δ-amino group in a segment derived from ornithine to a guanidino group. A represents hydrogen or an unsubstituted or substituted C 1 -C 12 alkyl group, wherein a substituent when the group is substituted represents a formyl group, a group of the formula R 1 R 2 CH- (where R 1 and R 2 independently represent C 1 -C 4 alkoxy or R 1 and R 2 together represent -OCH 2 CH 2 O-, -O(CH 2) 3 O- or -O(CH 2) 4 O-); L represents -O-(CH 2) a -NH-, -O-(CH 2) a -O-, -(CH 2) a -NH- or -(CH 2) a -O-, wherein a is an integer of 1 to 6 and the orientation of the linkage group L is the forward direction of each moiety in each structural formula; Y represents H, C 1-21 alkylcarbonyl, substituted C 1-4 alkylcarbonyl, unsubstituted or substituted C 3-7 cycloalkylcarbonyl, unsubstituted or substituted arylcarbonyl, or unsubstituted or substituted 5- or 6-membered heteroarylcarbonyl, wherein a substituent of the substituted C 1-4 alkylcarbonyl is selected from the group consisting of a halogen atom, hydroxyl, carboxyl, unsubstituted or substituted C 3-7 cycloalkyl, unsubstituted or substituted aryl, unsubstituted or substituted 5- or 6-membered heteroaryl, unsubstituted or substituted adamantyl and an unsubstituted or substituted cholesterol residue, wherein a substituent when the foregoing substituents are substituted may be C 1-4 alkyl, C 1-4 alkyloxy, hydroxyl, carboxyl, cyano, nitro, a halogen atom or mono- or di-C 1-4 alkylamino; m is an integer of 5 to 300; n is an integer of 5 to 1,000; and up to 80% of m amidino groups (C(=NH)NH 2) may be replaced by H, the method comprising a step of reacting a block copolymer represented by formula: wherein: A, L, Y, n and m are as defined for formula above, with N,N'-bis(tert-butoxycarbonyl)-1H-pyrazole-1-carboxamidine optionally in an inert solvent in order to convert a δ-amino group in a segment derived from ornithine to a guanidino group. A, L, Y, n and m are as defined for formula above, with N,N'-bis(tert-butoxycarbonyl)-1H-pyrazole-1-carboxamidine optionally in an inert solvent in order to convert a δ-amino group in a segment derived from ornithine to a guanidino group.