Abstract: Amount of sub-micron particles in fluid such as ultra pure water classified each range of particle dimensions are detected by a simple apparatus in which a light beam from a coherent light source(1) is converged (2) in such a manner that the light beam is focused in a stream (3) of particle-containing fluid, the light passed through the stream and diffracted by the particles is received by a photo-detector (4) which is positioned at an opposite side of the coherent light source with respect to the stream and substantially on an optical axis of the light beam, so that the number of particles in the stream is distinguished into plural ranges according to each frequencies of the signals emitted from the photo-detector and counting each numbers of the particles contained the ranges.