Imaging a cylindrical object, left and right viewpoint images are stored to a data memory. To calculate a diameter D of the cylindrical object, a pair of measurement points designated on outlines of the left viewpoint image and corresponding points that are set on outlines of the right viewpoint image in accordance with measurement points are used. While one of the measurement points is fixed on the outline, the other measurement point is scanned on the other outline, such that the distance between the pair of measurement points is minimized. The positions of the corresponding points are updated in synchronization with this. Whenever the corresponding points are updated, the diameter D is calculated, and a minimum value of the calculated diameters is determined as the diameter D of the cylindrical object.