Abstract: FRET measurement uses a FRET probe that includes a probe element X containing a donor fluorescent substance and a probe element Y containing an acceptor fluorescent substance and enables FRET to occur when the probe element X and the probe element Y approach to each other or bind together. The modulation frequency of laser light with which the FRET probe is irradiated is adjusted to an optimum modulation frequency that maximizes a difference between the phase difference of donor fluorescence emitted from the donor fluorescent substance with respect to intensity modulation of the laser light at the time when FRET occurs and the phase difference of donor fluorescence emitted from the donor fluorescent substance with respect to intensity modulation of the laser light at the time when FRET does not occur.