More than 80% of the pattern defects in a semiconductor manufacturing process are caused by the adhesion of particles on a wafer. In a production line, the particles larger than 0.2 μm in diameter on the wafer on which a Si0_2 thin film had been deposited, have to be detected. In this study, the intensity of light scattered by particles 0.2 μm in diameter on the wafer on which a Si0_2 thin film had been deposited, without being influenced by the thickness variation of the thin films, is detected using an experimental apparatus. The following results were obtained. (1) P-polarized laser light with the incident angle of 75° is suitable for the illumination. (2) A method of detecting the light scattered perpendicularly upward from the wafer is suitable. (3) The variation of the detected light intensity is 57% when the light scattered by a particle 0.2 μm in diameter on a wafer on which a 0-300-nm-thick Si0_2 thin film had been deposited, is detected. P-polarized laser light is suitable because the variation of the detected light intensity when a particle is illuminated by S-polarized laser light is 96%. The above-mentioned experimental results coincide fairly well with the results obtained by simulation.