A method for detecting a fluorescent particle dispersed and moving randomly in a sample solution, comprising: (a) a step for preparing a sample solution containing fluorescent particles, and (b) a step for calculating the number density of molecules of fluorescent particles present in the sample solution prepared in step (a); wherein, calculation of the number density of molecules of fluorescent particles in step (b) comprises: a step for moving the location of a photodetection region of an optical system in the sample solution using the optical system of a confocal microscope or multi-photon microscope, thereby obtaining a chronological light intensity data, a step for individually detecting a fluorescent particle by detecting a light signal released from the fluorescent particle present in the photodetection region from radiative decaying from excited states other than the triplet state while moving the location of the photodetection region in the sample solution, and a step for counting the number density of fluorescent particles detected during movement of the location of the photodetection region by counting the number of individually detected fluorescent particles and calculating the number density from this number, the size of the photo-detection region and the moving of its location, characterized in that the step for preparing a sample solution containing fluorescent particles further includes the preparation of a substance that promotes transition of the fluorescent particles from a triplet excited state to a singlet ground state, and in the step for individually detecting fluorescent particles by detecting a light signal from the individual fluorescent particle, the entry of a single fluorescent particle into the photodetection region is detected based on the shape of the detected chronological light signal. (a) a step for preparing a sample solution containing fluorescent particles, and (b) a step for calculating the number density of molecules of fluorescent particles present in the sample solution prepared in step (a); wherein, a step for moving the location of a photodetection region of an optical system in the sample solution using the optical system of a confocal microscope or multi-photon microscope, thereby obtaining a chronological light intensity data, a step for individually detecting a fluorescent particle by detecting a light signal released from the fluorescent particle present in the photodetection region from radiative decaying from excited states other than the triplet state while moving the location of the photodetection region in the sample solution, and a step for counting the number density of fluorescent particles detected during movement of the location of the photodetection region by counting the number of individually detected fluorescent particles and calculating the number density from this number, the size of the photo-detection region and the moving of its location, characterized in that the step for preparing a sample solution containing fluorescent particles further includes the preparation of a substance that promotes transition of the fluorescent particles from a triplet excited state to a singlet ground state, and in the step for individually detecting fluorescent particles by detecting a light signal from the individual fluorescent particle, the entry of a single fluorescent particle into the photodetection region is detected based on the shape of the detected chronological light signal.