A method for producing a stable isotope-labeled target peptide fragment in mass spectrometry, comprising the following steps (a) to (d): (a) expressing a DNA conjugate in a system having a stable isotope-labeled amino acid to thereby prepare a stable isotope-labeled protein, wherein the DNA conjugate comprises: a tandemly linked DNA in which two or more DNAs encoding one or more types of target peptide fragments are linked in tandem; a DNA encoding a peptide fragment for concentration measurement conjugated with the 5' end of the tandemly linked DNA; and a DNA encoding a peptide fragment for concentration measurement conjugated with the 3' end of the tandemly linked DNA differing in amino acid sequence from the peptide fragment for concentration measurement encoded by the DNA conjugated with the 5' end of the tandemly linked DNA; and wherein the peptide fragment for concentration measurement is a peptide fragment having 3 to 20 amino acid residues, having a C-terminal arginine residue or lysine residue, and having neither arginine nor lysine residues at a site other than the C terminus, and is different in amino acid sequence from naturally occurring proteins and their variants, and detectable by a liquid chromatograph-tandem mass spectrometer (LC/MS/MS); (b) subjecting the stable isotope-labeled protein to digestion by trypsin to prepare a stable isotope-labeled peptide fragment for concentration measurement and stable isotope-labeled target peptide fragments; (c) quantifying the stable isotope-labeled peptide fragment for concentration measurement using LC/MS/MS; and (d) calculating the concentration of the stable isotope-labeled target peptide fragments from the quantification value of the stable isotope-labeled peptide fragment for concentration measurement. (a) expressing a DNA conjugate in a system having a stable isotope-labeled amino acid to thereby prepare a stable isotope-labeled protein, wherein the DNA conjugate comprises: a tandemly linked DNA in which two or more DNAs encoding one or more types of target peptide fragments are linked in tandem; a DNA encoding a peptide fragment for concentration measurement conjugated with the 5' end of the tandemly linked DNA; and a DNA encoding a peptide fragment for concentration measurement conjugated with the 3' end of the tandemly linked DNA differing in amino acid sequence from the peptide fragment for concentration measurement encoded by the DNA conjugated with the 5' end of the tandemly linked DNA; and wherein the peptide fragment for concentration measurement is a peptide fragment having 3 to 20 amino acid residues, having a C-terminal arginine residue or lysine residue, and having neither arginine nor lysine residues at a site other than the C terminus, and is different in amino acid sequence from naturally occurring proteins and their variants, and detectable by a liquid chromatograph-tandem mass spectrometer (LC/MS/MS); (b) subjecting the stable isotope-labeled protein to digestion by trypsin to prepare a stable isotope-labeled peptide fragment for concentration measurement and stable isotope-labeled target peptide fragments; (c) quantifying the stable isotope-labeled peptide fragment for concentration measurement using LC/MS/MS; and (d) calculating the concentration of the stable isotope-labeled target peptide fragments from the quantification value of the stable isotope-labeled peptide fragment for concentration measurement.