PURPOSE: To enable measurement of speed, particle size, density, shape and the like of particles within a 3-D space with high resolution, by irradiating floating particles in a space with multiple slit beams varied in the color to take the particles with a camera.

CONSTITUTION: Particles contained in a fluid flows in a passage 1 along a direction (a) and the flow of the particles is irradiated with parallel beams 3. The parallel light 3 has slit beam of four different colors, red, yellow, blue and green overlapped in four layers with separate filters of respective colors. The particles look as such in color corresponding to the positions thereof and are observed from the direction vertical to the parallel light 3 with a color video camera 4. This enables discrimination of the positions of the particles vertical to the camera according to color, and hence, a moving speed of the particles can be determined by tracing the positions thereof for analysis. The speed component vertical to the slit beams of the particles is determined from changes in the color while individual particles are passing through the slit beams. Analysis required is performed using a computer 6.