A process for the manufacture of a polymer micelle composition encapsulating a polypeptide or protein and composed of a block copolymer comprising hydrophilic segments composed of polyethylene glycol, and hydrophobic segments composed of a polyamino acid selected from among acidic amino acids, their hydrophobic derivatives and mixtures of acidic amino acids and their hydrophobic derivatives, wherein the polypeptide or protein is encapsulated in the polymer micelle comprising a step of mixing the block copolymer with the protein or polypeptide, and adjusting the pH of the mixture to a pH different from the isoelectric point (pI) of the protein or polypeptide to encapsulate the protein or polypeptide into the hydrophobic core region of micelles composed of the block copolymer, wherein the pI of the protein or polypeptide, the isoelectric point (pI') of the acidic amino acid and/or its derivative in the hydrophobic segments of the block copolymer, and the pH that is different from the pI, are in the relationship: pI > pH > pI' such that the hydrophobic segments of the block copolymer are negatively charged at the pH while the protein or polypeptide is positively charged, or are in the relationship: pI'> pH > pI such that the hydrophobic segments of the block copolymer are positively charged at the pH while the protein or polypeptide is negatively charged and wherein the pH has a difference of at least 1 from the pI of the protein or polypeptide, wherein the block copolymer has the following formula or. or wherein, R 1 and R 3 each independently represent hydrogen or a lower alkyl group either unsubstituted or substituted with an optionally protected functional group, R 2 represents hydrogen, a saturated or unsaturated C 1 -C 29 aliphatic carbonyl group or an arylcarbonyl group, R 4 represents hydroxyl, a saturated or unsaturated C 1 -C 30 aliphatic oxy or aryl-lower alkyloxy group, R 5 represents -O- or -NH-, R 6 represents hydrogen, phenyl, -(CH 2) 4 -phenyl, C 4 -C 16 alkyl either unsubstituted or substituted with an amino group or carboxyl group, or benzyl, R 7 represents methylene, n represents an integer of 10-2500, x represents an integer of 10-300, m represents an integer of 0-300 (with the proviso that when m is present, the (COCHNH) units and (COR 7 CHNH) units are random, R 6 may be freely selected for each amino acid unit in one block copolymer and is randomly present, but hydrogen as R 6 constitutes less than 60% of the total R 6), y represents an integer of 1 or 2, L 1 represents a linking group selected from the group consisting of -NH-, -O-, -O-Z-NH-, -CO-, -CH 2 -, -O-Z-S-Z- and -OCO-Z-NH- (where each Z independently represents a C 1 -C 6 alkylene group), and L 2 represents a linking group selected from among -OCO-Z-CO- and -NHCO-Z-CO- (where Z is a C 1 -C 6 alkylene group). wherein the pI of the protein or polypeptide, the isoelectric point (pI') of the acidic amino acid and/or its derivative in the hydrophobic segments of the block copolymer, and the pH that is different from the pI, are in the relationship: pI > pH > pI' such that the hydrophobic segments of the block copolymer are negatively charged at the pH while the protein or polypeptide is positively charged, or are in the relationship: pI'> pH > pI such that the hydrophobic segments of the block copolymer are positively charged at the pH while the protein or polypeptide is negatively charged and wherein the pH has a difference of at least 1 from the pI of the protein or polypeptide, wherein the block copolymer has the following formula or. or wherein, R 1 and R 3 each independently represent hydrogen or a lower alkyl group either unsubstituted or substituted with an optionally protected functional group, R 2 represents hydrogen, a saturated or unsaturated C 1 -C 29 aliphatic carbonyl group or an arylcarbonyl group, R 4 represents hydroxyl, a saturated or unsaturated C 1 -C 30 aliphatic oxy or aryl-lower alkyloxy group, R 5 represents -O- or -NH-, R 6 represents hydrogen, phenyl, -(CH 2) 4 -phenyl, C 4 -C 16 alkyl either unsubstituted or substituted with an amino group or carboxyl group, or benzyl, R 7 represents methylene, n represents an integer of 10-2500, x represents an integer of 10-300, m represents an integer of 0-300 (with the proviso that when m is present, the (COCHNH) units and (COR 7 CHNH) units are random, R 6 may be freely selected for each amino acid unit in one block copolymer and is randomly present, but hydrogen as R 6 constitutes less than 60% of the total R 6), y represents an integer of 1 or 2, L 1 represents a linking group selected from the group consisting of -NH-, -O-, -O-Z-NH-, -CO-, -CH 2 -, -O-Z-S-Z- and -OCO-Z-NH- (where each Z independently represents a C 1 -C 6 alkylene group), and L 2 represents a linking group selected from among -OCO-Z-CO- and -NHCO-Z-CO- (where Z is a C 1 -C 6 alkylene group).