PURPOSE: To measure large-small particles at a time by converging light which has a small scatter angle on the same laser optical axis through a lens which has long focal distance and converging light which has a large scatter angle through a lens which has short forcal distance.

CONSTITUTION: A detector 5 which is arranged on the focal plane of the lens 4 with the short focal distance measures the light 3a which is scattered by a particle group 2 at the large scatter angle. The light 3B which has the small scatter angle is passed as it is since the light is close to the optical axis 8. This light 3b is passed through the lens 6 to have a focal plane at a distance L3 (=composite focal distance of lenses 4 and 6) which is an integral multiple of the focal distance L1 of the lens 4. For example, when L1=60mm, the focal distance of the lens 6 is 50mm, and the distance between the lenses 4 and 6 is 120mm, L3=300mm. Namely, the light 3b is enlarged to have its focus on a detector 7, so that the light is measured effectively. Thus, scattered light beams from the the particle group 2 which have small-large angles are measured by the detectors 5 and 7 at the same time.