1. A method of inspecting a surface of a workpiece by using a surface inspecting apparatus provided with lighting means for illuminating the workpiece surface to be inspected with detection light, detecting means for receiving and detecting the light reflected from the workpiece surface, and an optical system for causing said reflected light to converge on a light-receiving surface of said detecting means, comprising the following steps of: successively displacing said surface inspecting apparatus along the workpiece surface; obtaining an image of the workpiece surface to be inspected by receiving and detecting said reflected light by said detecting means; holding said image; binary digitizing said held image; determining whether or not the workpiece surface to be inspected is planar; processing the digitized image for dilation and erosion only when the workpiece surface to be inspected is non-planar; extracting isolated dark areas from said digitized image; and determining whether each of said isolated dark areas represents dust, based on the size of each of said isolated dark areas.