PURPOSE: To use a compact solid laser beam source for light scattering of a sample by subtracting a smoothed scattered light signal by a smoothed laser monitor signal, and calculating the output ratio of scattered light to incident light of the sample.

CONSTITUTION: A waveform generator 5 modulates the frequency and amplitude of a laser control power source 4, and a solid laser 3 emits a modulated laser beam 10 to a sample fluid in a sample cell 1. A plurality of scattered light detectors for collimation surrounding the cell 1 at different angles detect the scattered light of the sample, and the scattered light signals are averaged by analog, digital filters 6, 7 to smooth the scattered light fluctuation resulted from modulation. On the other hand, a part of the beam 10 is reflected 11 and incident on a laser monitor 12, and the output fluctuation of this monitor signal is smoothed 6, 7. This is multiplied 8 and inputted to a computer 9, which subtracts the smoothed scattered light signal by the smoothed monitor signal. The output ratio of scattered light to incident light is calculated every measuring angle to measure the light scattering of the sample.