A fluorescence detecting device generates a modulation signal for modulating an intensity of laser light and modulates the laser light by using the modulation signal, when receiving fluorescence emitted by a measurement object irradiated with laser light emitted from a laser light source unit. The fluorescence detecting device obtains a fluorescent signal of the fluorescence emitted by the measurement object irradiated with the laser light and calculates, from the fluorescent signal, the phase delay of the fluorescence with respect to the modulation signal. At the time, the fluorescence detecting device controls the frequency of the modulation signal so that the value of the phase delay comes close to a preset value. The fluorescence detecting device calculates the fluorescence relaxation time of the fluorescence emitted by the measurement object by using a phase delay obtained under the condition of frequency of the modulation signal at the time when the control is settled.