A projection height measuring method measures the height of a projection having a conical shape, formed on the surface of a workpiece. The area having the projection is imaged at an angle of depression. A projection area is extracted by classifying an area in the image into a bright area, a dark area and an intermediate area. From these, an area composed of the bright area and the dark area indicates the projection area, and the intermediate area indicates the work area. The length of the bottom surface diameter the length of a generatrix of the projection area are determined. The height of the projection is calculated based on the two lengths and the angle of depression.