Kit for detecting antigen or measuring its amount

A method for detecting antigen or measuring its amount in a sample, comprising sequentially performing steps (1) to (3):

• (1) bringing a sample into contact with a complex composed of a polypeptide including an antibody light chain variable region and a polypeptide including an antibody heavy chain variable region in the presence of a protein M fragment labeled with a fluorescent dye, wherein the protein M fragment is a fragment having a binding ability to the complex;
• (2) measuring the fluorescence intensity of the fluorescent dye;
• and
• (3) judging presence of an antigen in the sample from the fluorescence intensity or calculating an antigen level in the sample from the fluorescence intensity.