An infrared fluorescent particle comprising a functional group or a substance that is capable of binding to an analyte, wherein fluorescence at infrared wavelength is emitted from the particle upon exposure of the particle to excitation light at infrared wavelength. The infrared fluorescent particle of the present invention is capable of binding to the analyte. Due to a high penetration of the fluorescence and the excitation light into biological substances; the infrared fluorescent particle of the present invention can reduce an influence of luminescence, light absorption or light scattering which is occurred due to the analyte and the surrounding substances.