PURPOSE: To improve the accuracy of measuring the concn., quantity, numerical density, etc., of the particle to be measured by irradiating a sample with at least two beams of stimulating light varying in wavelength and calculating the resulted unknown quan tity of the particle to be measured.

CONSTITUTION: Laser light sources 1a, 1b output laser beams of wavelengths λ1, λ2, respectively. These laser beams are modulated as intermittent beams of a specified frequency by light choppers 2a, 2b and are introduced to an optical path 8. The stimu lating light of the wavelength λ2 generates only the acoustic signal of a negligible level to the particle to be measured when the sample is irradiated with this light and, therefore, the generated acoustic detection signal P 2 can be regarded as only the signal nearly relating to the disturbing particle. The signal component P 1' of the disturbing particle in the acoustic signal P 1 generated by the irradiation with the wavelength λ1 is, therefore, determined from the signal P 2 by using conversion data. Any of the concn., quantity, numerical density, etc., of the particle tt be measured, from which the influence of the disturbing wave is substantially eliminated, is obtd. from the difference between the signals P 1 and P 1'.