Abstract: Disclosed is a method and apparatus for measuring particles in a fluid by irradiating the fluid containing the particle with a laser beam and deriving particle characteristics such as diameter and size distribution from the intensity of the light scattered by the particles. Based on the value of the output of a photomultiplier used to detect the scattered light, it is determined whether a particle is a fine particle, which is a particle with a photoelectron pulse count that does not exceed a prescribed value, or a large particle, and these particles are counted separately. Fine particles are processed using photon counting, and large particles are processed by an analog process, in which case the threshold values used to discriminate particles are varied in accordance with pre-stored data on the refractive index of each fluid.