A compact optical system for a particle analyzer and particle analyzer using same are provided. The optical system for a particle analyzer of the present invention comprises a light source, an irradiation optical system for irradiating particles passing through a flow cell with light from the light source, a photodetector for receiving the scattered light from the particles, a light shielding member for blocking the direct light from the light source from impinging the photodetector, and a detecting lens for directing the scattered light toward the photodetector, wherein the irradiation optical system forms a first focus that focuses the light from the light source on the particle passing through the flow cell, and forms a second focus that focuses the light from the light source at a position between the detecting lens and photodetector, and disposes the light shielding member at the position of the second focus.