Optical reflectance method is studied for surface roughness measurement. In order to measure the intensity of the light reflected by a surface at a fixed distance, here employed is an instrument named 'optomicrometer', which is originally used to measure the vertical displacement of a surface. Also discussed is the relation between the relative intensity of reflected light and the surface roughness parameters. The optical wave theory shows that a surface is classified into one of four models according to whether it is rough or smooth and whether its finish is one-directional or equidirectional lay and that the reflected light intensity : is determined by its mean deviation for a smooth surface and by its mean slope for a rough one. Experiments are carried out with various kinds of finished surfaces of which the arithmetical mean deviation ranges from 0.02 to 2.1μm. The results support the theory as well as the proposed method so that the usefulness of optical reflectance method is certified.