It is examined whether the principle of reflectometry, developed to measure the thickness of a thin layer oh a surface, is applicable to detecting quantitatively the amount of fine particles deposited on a substrate, It is found that the relative difference of parallel and perpendicular components of the laser beam reflected on the surface is proportional to the particle coverage if experimental conditions are suitably selected, and that this principle is able to be used to detect with high sensitivity in-situ the amount of particles.