PURPOSE: To prevent the generation of mechanical vibration and temp. drift, by constituting the titled apparatus from a plurality of interference filters for allowing the passage of only lights which are mutually different in a wavelength region and a light receiving element for receiving the lights passed through said filters and outputting electric signals corresponding to the light intensities thereof.

CONSTITUTION: The light 2 from a light source 1 is allowed to project a particle stream A through a light transmitting optical path 7 while condensed and scattered and attenuated. The light 2 is further propagated through a light receiving optical fiber 11 and separated into its spectral components as the rear end part 11B thereof while the separated lights are respectively passed through interference filters 5 having mutually different characteristics arranged to the rear end 11C of the light receiving optical fiber 11 and respectively received by light receiving elements 8 while converted to lights each having a narrow wavelength region. The light receiving elements 8 output the electric signals corresponding to the light intensities thereof to an amplifier 12. Therefore, it is unnecessary to change over the interference filters 5 as is conventional and no mechanical vibration is generated. In addition, the particle stream A and the light receiving elements 8 can be separated and a problem such as temp. drift is not generated.