PURPOSE: To obtain a device that does not depend on reading based on the keenness of each human's feeling by forming a diffraction limitation sport along a display region in a gap.

CONSTITUTION: Light beams 52 from a laser diode are converged 32 and a diffraction limitation spot with a size that is approximately equal to the diameter of a single particle to be scanned in a gap 47 of a slide 44 is formed along the longitudinal direction of a display region. A photo detector 34 detects the level of light that is attenuated by a particle in the gap 47 and is moved in the direction of an arrow A, thus obtaining a signal that indicates the size and the number of particles. As a result, beams 52 pass through a particle 54 as shown in a figure a. and are directed in the direction of an arrow B. Then, when the slide 44 is scanned in the direction of an arrow C, the beams 52 are partially attenuated by the particles 54 in a light path (Figure a2), the output of the detector 34 decreases to a maximum attenuation point (Figure a3). and particle attenuation decreases and returns to the nominal level of the beams 52 that are not blocked at all. Therefore, by calculating the distance between the continuous readings of the level, the size of particles can be measured accurately.