A method for measuring histone methyltransferase activity in a sample, comprising the steps of: (a) providing a substrate compound represented by the following general formula: R 1 -X-K-R 2 or a salt thereof (wherein R 1 represents a hydrogen atom or a protecting group for an amino terminus; X represents a peptide consisting of 0 or 1 or more amino acid residues; K represents a lysine residue; and R 2 represents a dye label linked via an amide bond to the carbonyl terminus of a lysine residue), wherein the cleavage of the amide bond by peptidase changes the fluorescence property or chromogenic property of the dye label, and the methylation of the ε amino group of the lysine residue by the histone methyltransferase decreases susceptibility to peptidase; (b) contacting the substrate compound represented by the general formula or a salt thereof with a sample under conditions required for methylation reaction by the histone methyltransferase; (c) exposing the substrate compound or a salt thereof to peptidase after the step (b); (d) measuring the degree of the change of the fluorescence property or chromogenic property of the dye label after the step (c) to calculate the degree of the increase of the methylation level of the substrate compound or a salt thereof, based on the degree of the decrease of the cleavage activity of the peptidase that uses the substrate compound or a salt thereof as a substrate; and (e) evaluating the degree of the increase of the methylation level in the step (d) as the degree of histone methyltransferase activity in the sample. (a) providing a substrate compound represented by the following general formula: R 1 -X-K-R 2 or a salt thereof (wherein R 1 represents a hydrogen atom or a protecting group for an amino terminus; X represents a peptide consisting of 0 or 1 or more amino acid residues; K represents a lysine residue; and R 2 represents a dye label linked via an amide bond to the carbonyl terminus of a lysine residue), wherein the cleavage of the amide bond by peptidase changes the fluorescence property or chromogenic property of the dye label, and the methylation of the ε amino group of the lysine residue by the histone methyltransferase decreases susceptibility to peptidase; R 1 -X-K-R 2 or a salt thereof (wherein R 1 represents a hydrogen atom or a protecting group for an amino terminus; X represents a peptide consisting of 0 or 1 or more amino acid residues; K represents a lysine residue; and R 2 represents a dye label linked via an amide bond to the carbonyl terminus of a lysine residue), wherein the cleavage of the amide bond by peptidase changes the fluorescence property or chromogenic property of the dye label, and the methylation of the ε amino group of the lysine residue by the histone methyltransferase decreases susceptibility to peptidase; (b) contacting the substrate compound represented by the general formula or a salt thereof with a sample under conditions required for methylation reaction by the histone methyltransferase; (c) exposing the substrate compound or a salt thereof to peptidase after the step (b); (d) measuring the degree of the change of the fluorescence property or chromogenic property of the dye label after the step (c) to calculate the degree of the increase of the methylation level of the substrate compound or a salt thereof, based on the degree of the decrease of the cleavage activity of the peptidase that uses the substrate compound or a salt thereof as a substrate; and (e) evaluating the degree of the increase of the methylation level in the step (d) as the degree of histone methyltransferase activity in the sample.