A method for determining whether a heated sample has reached a predetermined temperature, comprising the steps of: a) mixing an albumin-containing sample with a peptide reagent comprising a peptide for temperature determination, wherein said peptide for temperature determination: i) has a dissociation constant (K d) for albumin of 500 μM or more, ii) comprises a linker peptide of 2 to 130 amino acid residues; wherein said linker peptide comprises at least one amino acid residue selected from the group consisting of aspartic acid, glutamic acid, asparagine, serine, glycine, leucine, alanine, proline, and lysine residues, and a first labeling substance and a second labeling substance selected from: a fluorescent substance and a quencher that quenches fluorescence from the fluorescent substance; a first fluorescent substance and a second fluorescent substance serving as an acceptor which receives energy from the first fluorescent substance; and a bioluminescent substance and a fluorescent substance serving as an acceptor which receives energy from the bioluminescent substance, wherein said at least one amino acid residue is located between the first labeling substance and the second labeling substance, and wherein the first labeling substance and the second labeling substance are separated from each other by cleavage of the peptide at the predetermined temperature, thereby producing a detectable optical change; b) heating the mixture; c) detecting an optical change of the mixture, wherein the optical change is a change in wavelength or fluorescence generation produced by the first labeling substance and the second labeling substance due to the cleavage of the peptide for temperature determination; and d) determining whether the temperature of the mixture has reached a predetermined temperature based on the detection result of the optical change, wherein it is determined that the temperature of the mixture is equal to or higher than the predetermined temperature when the optical change has been detected. a) mixing an albumin-containing sample with a peptide reagent comprising a peptide for temperature determination, wherein said peptide for temperature determination: i) has a dissociation constant (K d) for albumin of 500 μM or more, ii) comprises a linker peptide of 2 to 130 amino acid residues; wherein said linker peptide comprises at least one amino acid residue selected from the group consisting of aspartic acid, glutamic acid, asparagine, serine, glycine, leucine, alanine, proline, and lysine residues, and a first labeling substance and a second labeling substance selected from: a fluorescent substance and a quencher that quenches fluorescence from the fluorescent substance; a first fluorescent substance and a second fluorescent substance serving as an acceptor which receives energy from the first fluorescent substance; and a bioluminescent substance and a fluorescent substance serving as an acceptor which receives energy from the bioluminescent substance, wherein said at least one amino acid residue is located between the first labeling substance and the second labeling substance, and wherein the first labeling substance and the second labeling substance are separated from each other by cleavage of the peptide at the predetermined temperature, thereby producing a detectable optical change; i) has a dissociation constant (K d) for albumin of 500 μM or more, ii) comprises a linker peptide of 2 to 130 amino acid residues; wherein said linker peptide comprises at least one amino acid residue selected from the group consisting of aspartic acid, glutamic acid, asparagine, serine, glycine, leucine, alanine, proline, and lysine residues, and a first labeling substance and a second labeling substance selected from: a fluorescent substance and a quencher that quenches fluorescence from the fluorescent substance; a first fluorescent substance and a second fluorescent substance serving as an acceptor which receives energy from the first fluorescent substance; and a bioluminescent substance and a fluorescent substance serving as an acceptor which receives energy from the bioluminescent substance, wherein said at least one amino acid residue is located between the first labeling substance and the second labeling substance, and wherein the first labeling substance and the second labeling substance are separated from each other by cleavage of the peptide at the predetermined temperature, thereby producing a detectable optical change; b) heating the mixture; c) detecting an optical change of the mixture, wherein the optical change is a change in wavelength or fluorescence generation produced by the first labeling substance and the second labeling substance due to the cleavage of the peptide for temperature determination; and d) determining whether the temperature of the mixture has reached a predetermined temperature based on the detection result of the optical change, wherein it is determined that the temperature of the mixture is equal to or higher than the predetermined temperature when the optical change has been detected.