PURPOSE: To shorten the size of the device in the direction of an optical axis and thereby miniaturize the constitution of an optical system by adopting a constitution wherein irradiating and light-receiving systems are bent at an angle of 180° by using a plane reflecting mirror.

CONSTITUTION: Irradiating light beams from a light source 22 which pass through a lens system 23 are bent at an angle of 90° approximately by a plane mirror 20 and condensed into an irradiation region P to irradiate sample air, and further are bent by a plane mirror 21 and absorbed in a phototrap chamber 19 to which photoabsorption processing is applied. Scattered lights generated in the irradiation region P by the particulates in the sample air are bent at an angle of 90° approximately by a plane mirror 28 and condensed into the light-receiving surface of a photoelectric transducer 30 by a lens system 31. The component of the scattered lights which is not projected directly on the plane mirror 28 is bent at an angle of 90° by a plane mirror 29 and absorbed in a phototrap chamber 25. An irradiating chamber 18 and a light-receiving chamber 24 are set in parallel to the path of the sample air wherein the irradiation region P is set, while the optical axes of the irradiating light, scattering light, etc. are bent by the plane mirrors 20 and 28, and thus the constitution of the optical system can be miniaturized.