The present invention is to provide a method of quickly detecting an antigen at an arbitrary concentration in an antigen sample, without a multi-stage examination of the concentration ratio between a detection reagent and an antigen to be detected, particularly when the concentration of the antigen in the sample is unknown, in the method of detecting an antigen using fluorescence correlation spectroscopy (FCS) or fluorescence cross-correlation spectroscopy (FCCS). By preparing (1) a series to which only a detection reagent is added and (2) a series to which an antigen and the detection reagent are added to achieve a maximum trimer concentration, and by performing a fluorescence spectroscopic analysis, the presence or absence of the antigen in the detection sample is quickly detected by the presence or absence of a trimer detection signal from a detector in the cases of (1) and (2), in a method of detecting an antigen by FCS or FCCS using as a detection reagent a fluorescent-labeled intact antibody or fluorescent-labeled antibody fragment targeted to an epitope of the antigen to be detected, and a non-fluorescent-labeled intact antibody or fluorescent-labeled intact antibody or fluorescent-labeled antibody fragment targeted to another epitope of the antigen.