Fluorescence detection device employed in a flow site meter emits laser light intensity-modulated by a modulation signal and acquires the fluorescence signal of fluorescence emitted from a measurement object passing through a measurement point of the laser light. The device generates the reference signal, separately from the modulation signal, the reference signal having a frequency different from the frequency of the modulation signal and having a phase synchronized with a phase of the modulation. The device determines fluorescence relaxation time of the measurement object from the fluorescence signal using the reference signal.