Laser sources produce two light beams which are different in frequency and at least one of which is a pulse beam. The two light beams are subjected to sum frequency mixing in a non-linear optical element and a resultant sum frequency beam is applied to a specimen as an exciting pulse beam. A pulse beam separated from the sum frequency beam and synchronized therewith is detected by a photodetector. A measuring means measures a waveform of fluorescence light emitted from the specimen using an output of the photodetector as a measurement starting reference signal. According to one embodiment, the device operates in a single photon counting mode and the measuring means counts repeatedly an elapsed time from the detection of the measurement start reference signal to the detection of the fluorescence light for every divided section of the elapsed time.