A photon correlator comprises a plurality of sampling gates 11a-11e which are open during different periods of time; a plurality of memories 12a-12e each provided corresponding to each of the plurality of sampling gates 11a-11e for storing data corresponding to the number of photons; and a data processing control section for reading out the data stored in the memories 12a-12e, and performing a correlation calculation by means of software. The mechanism of the hardware comprising the sampling gates 11a-11e and memories 12a-12e enables high-speed writing of data in the memories and real-time read out of the data. In addition, the software performs correlation calculations in parallel with the above processing. Accordingly, the particle sizes and diffusion coefficient of particles in a fluid can be obtained at high speed under various conditions.