Abstract: An apparatus for measuring the diameter of the cutting portion of a drill comprising supporting-rotating means for supporting the drill and rotating the drill about its axis; an image sensor having a light receiving surface parallel with the drill and composed of a plurality of sensor elements, the sensor elements being arranged in the light receiving surface in a row at right angles with the drill; optical means for forming parallel rays propagating toward the cutting portion of the drill and for forming a silhouette of the drill cutting portion on the light receiving surface of the image sensor; silhouette width recognizing means for receiving from the image sensor a plurality of output patterns representing varying silhouette widths while the drill is rotated through a predetermined angle and for storing the output pattern corresponding to the largest silhouette width as selected from among the plurality of output patterns; and reading-displaying means for reading out the stored data from the silhouette wi