Theoretical researches and measurement of the mean diameter and the diameter distribution of particles, such as low-concentration water droplets appearing in wet steam turbines, have been done by using an optical measurement apparatus with a He-Ne laser developed by the authors. In the design of experimental system, an anti-Gaussian filter is used to eliminate the effect of Gaussian beam and also a Fourier Transformation lens of high precision is applied to eliminate the spherical aberration In addition, a photosensitive detector is manufactured. By meana of these three deyio^ it is possible to greatly inorease the measurement aoouraoy. This apparatus is oa!ibra1)ep' by directly measuring diameters of glass beads suspended in water. The results are-compared with those obtained by miorophotographio statistical method and with those obtained by using a Malvern instrument. The experimental results indicate that this! light scattering apparatua is oharaoterized by high precision, wide application area andi broad measurement range of particle size (5~200 μm).