PURPOSE: To facilitate the optical axis adjustment of an optical system and to reduce the size of the device by coupling at least parts of an illumination optical system and a photodetection optical system with each other integrally by a coupling member.

CONSTITUTION: A laser light source 21, a projection port 21a, a flow cell 23, cylindrical lenses 22 and 24 (arranged coaxially with each other), and objective lenses 26 and 27 (lens 27 provided at right angles to a laser beam made incident on the cell 23) are fixed mutually by the coupling part 28 made of brass, etc., having a small coefficient of thermal expansion. The laser beam projected on the optical axis 01 from the projection port 21a of the light source 21 passes through the lenses 22 and 24 to irradiate particles to be inspected which flow in the distribution part 23a in the cell 23 as an elliptic spot. The forward scattered light which is generated at this time is photodetected by a photodetector through a stopper 25 and the lens 26. The sideward scattered light and fluorescent light are photodetected by a photodetector through the lens 27 on the optical axis 02. Thus, the optical axes 01 and 02 need not be adjusted at all after the assembly and the device is reduced in size on the whole.