Abstract: A particle size measuring device for effecting with regard to particles present in a fluid substance the simultaneous measurement in situ of two parameters thereof; namely, particle size distribution and volumetric density. The particle size measuring device comprises a laser light source (14), a laser light conveying device (40), a collimator, a first focusing device (64), a defined sample path (74) through which particles to be measured pass, a second focusing device (78) and a detector device (82). The mode of operation of the particle size measuring device is such that the laser light from the laser light source (14) is conveyed by means of the laser light conveying device (40) to the collimator. After being collimated the laser light is transmitted to the first focusing device (64) whereby the laser light is focused and is then made to pass across the sample path (74).