A fine particle detector includes a light emitting system letting light from a light source pass through a phase difference element and focusing the light on a sample flow through which fine particles flow. When the direction of the sample flow is an X-axis direction, the light is emitted to the sample flow in a Z-direction, and a ZX-plane is orthogonal to a Y-direction, then the phase difference element has a plurality of regions divided in the Y-axis direction and causes a phase difference between wavefronts of the light passing through the plurality of regions.