Abstract: APPARATUS FOR CALIBRATING the position of the support platform of a wafer cassette holder with respect to a robot blade. The apparatus comprises a housing including an open front portion which permits the convenient extending of the robot blade into the housing when the apparatus is placed on the support platform. The housing further comprising detecting and measuring means for detecting the presence underneath of a wafer held by the extended robot blade, and measuring the spacing between the detecting and measuring means and the wafer; and a display panel coupled to the detecting and measuring means for displaying spacing information which indicates to a user the moment when the spacing between the wafer and the detecting and measuring means reaches at least one predetermined spacing value including a desired spacing value.