The growing portion of a single crystal 1 grown by the pull method is recorded and the image signal is output to a digitizing circuit 3 that converts the image signal into binary data. A memory device 4 stores the digitized images and the point P at the boundary between dark and light is detected by scanning the stored digitized images starting from the scanning-start pixel and proceeding in the direction parallel to the direction of single crystal pulling. The diameter D of the growing portion of the single crystal is determined based upon the boundary P, and the scanning-start pixel for the current operation is set at the pixel that is separated from the boundary Pb in the preceding operation by a preset number of pixels d in the opposite direction of the scan.