To reduce the influence of air bubbles and dust in measuring scattered light beams on an automatic analyzer. A light source emits a light beam with a first wavelength 18a on a shorter wavelength side and a light beam with a second wavelength 18b on a longer wavelength side, and then, transmitted light beams 19a and 19b and scattered light beams 21a and 21b are received. Noise is estimated from the ratio between the intensities of the transmitted light beams with the first wavelength and the second wavelength and a change in the amount of the scattered light beam with the second wavelength. Then, the estimated noise is subtracted from a change in the amount of the scattered light beam with the first wavelength, so that noise due to air bubbles and dust is reduced.