To provide a method by sandwich immunoassay capable of detecting troponin I with higher sensitivity, when detecting troponin I in a biosample by using a specific antibody for recognizing troponin I.

In this method for measuring troponin I, troponin I originated in a biosample is brought into contact with the first antibody recognizing an amino acid sequence of No.41-49 of troponin I as epitope and labeled with a labeling material, in the presence of troponin C, to thereby form a complex of troponin I, troponin C and the first antibody, and the labeling material of the first antibody forming the complex is measured.