To make it easy for an operator to judge whether calibration work is installed with an attitude suited to a height range of a measuring object set by a user.

In a screen set for calibration processing, the operator inputs lower and upper limit values of a height measuring range in input boxes 204 and 205 and then selects the attitude of the calibration work by some of buttons 206-208. When the operator adjusts a calibration plate thereafter so that it may take the selected attitude, an angle indicating an inclination of the surface of the calibration work to a plane (reference plane) intersecting perpendicularly the optical axis of a camera for a front view is calculated by processing using an image formed by this camera. Based on the calculated angle, moreover, whether or not the attitude of the calibration work is suited to the height measuring range is determined, and the result of determination is displayed on the set screen.