A flow cell 2, a composite lens 15, and first to third detectors 16 to 18 are arranged in this sequence in the light pathway of light emitted from a laser light source 1. The composite lens 15 is configured by a convex lens 15a, and lens elements 15b and 15c. These lenses have different focal lengths. When the posture of the composite lens is correct, light impinging on the convex lens forms an image on the first detector. Therefore, the positioning of the composite lens is enabled. When particles in the flow cell are irradiated with laser light, forward scatter is produced. Forward small angle scatter having a small scattering angle impinges on the first lens element 15b to be collected thereby, and is then received by the second detector 17. Forward large angle scatter having a large scattering angle impinges on the second lens element 15c in the outermost periphery to be collected thereby, and is then received by the third detector 18 which is remotest from the composite lens.