Abstract: An object of the present invention is to provide a method for measuring tryptase activity in a blood sample accurately and rapidly by a convenient operation in order to accurately evaluate the state of a disease whose state involves mast cells. The present invention enables tryptase activity in a blood sample to be directly measured without the pretreatment, such as purification or concentration, of the blood sample, using a substrate for measuring tryptase activity, comprising a tripeptide C-terminally linked through a peptide bond to a dye label, selected from the following formulas (1) to (3): (1) Lys-Ala-Arg-X, (2) Ala-Ala-Arg-X, and (3) Abu-Ala-Arg-X (wherein X represents a dye label whose fluorescence characteristics or color development characteristics change upon the cleavage of the peptide bond with Arg, and Abu represents 2-aminobutyric acid).