The present invention provides a method of quickly and accurately detecting and/or assaying an antigen using fluorescence correlation spectroscopy (FCS), which involves a fluorescence-labeled antibody fragment and a non-fluorescence-labeled intact antibody that form a complex with the antigen. There is a significant difference in diffusion rate between the fluorescence-labeled antibody fragment not bonded to the antigen and the complex formed by the the fluorescence-labeled antibody fragment, the antigen, and the non-fluorescence-labeled intact antibody, and this diffusion rate can be determined using FCS. The antigen can be an antigenic protein, such as an abnormal prion or a harmful protein contained in a food material. According to this method, antigens over a wide scope can be assayed regardless of the shape or molecular weight.