PURPOSE: To output the average diameter and the concentration distribution of particles in a region to be measured as tomogram images by projecting white laser light from a laser light source, splitting the light into three primary colors in detecting parts, and scanning a region to be measured only once with the laser light.

CONSTITUTION: The white laser light from a laser light source 14 is deflected through a plurality of deflecting devices 17a - 17(n - 1) which are arranged around a region to be measured 10 and projected into a plurality of detecting parts 24a - 24n which are arranged around the region to be measured 10. The white laser light which is emitted from the laser light source 14 or each of the deflecting devices 17a - 17(n - 1) is received with each of the detecting parts 24a - 24n and divided into the three primary colors of red, green and blue. The detected signal of each color is inputted into a computer. In this way, a plurality of computer tomogram images of the region to be measured 10 and the particle diameter in the region to be measured 10 can be measured at the same time by scanning the region with the laser light only once.