PURPOSE: To eliminate irregularities at a visual inspection thereby to correctly measure a diameter of a circle even when an obstructing circle overlaps, by measuring a distance from a central position to a boundary of a photographed image of the circle in every direction and calculating a changing coefficient.

CONSTITUTION: An image photographed by an image pickup device 1 such as a CCD camera or the like is stored in a memory device 2. Only an image of an obstructing circle is extracted from the photographed image by a binarization device 3. A distance-measuring device 4 measures a distance from a central position to a boundary of each obstructing circle in every direction and inputs to a diameter-determining device 5. Inn the diameter-determining device 5, first, a changing coefficient (standard deviation/average value) of a group of the distances is calculated. If the changing coefficient is larger than a preset value, a distance separated from the average value by the standard deviation is removed as an abnormal value and a changing coefficient of the remaining values of distances is calculated. The process is repeated until the changing coefficient becomes smaller than the preset value. Twice the distance left to the very end is determined and output as a diameter of the circle.