In a light scattering liquid-borne particle counter, it is known that the scattered light caused by the sample liquid itself contributes to the noise. Generally, fluctuation of scattered light from the sample liquid itself is superimposed on the scattered light of the object particles detected by the particle counter. The frequency of an output signal from the light detecting element that detected the scattered light from the sample liquid itself is higher than the frequency of an output signal from the light detecting element that detected the scattered light from the particles. The noise due to the scattered light from the sample liquid itself is attenuated by processing the output signal from the light detecting element with low pass filters.