PURPOSE: To make the prevention of measuring errors possible by interposing contactors which interlock to measuring elements but are electrically insulated from each other between an object to be measured and the mesuring elements and displaying the contact states between the contactors and the object electrically.

CONSTITUTION: Contactors which interlock to measuring elements but are electrically isolated are interposed between an object to be measured and the measuring elements, and the contact states between the contact parts thereof and said object are electrically displayed by connecting an external electric power source and a display device, whereby whether the measurement states are normal or not is decided. For example, one or plural pieces of home position contactors 4, 4' which are fixed and held in prescribed positions and one set or plural sets of moving contactors 3, 3' which are slidable vertically from the prescribed positions are used as shown in the figure, and the home position contactors are fixed respectively in reference positions. Measurement is carried out by grasping an object 2 to be measured with the respective sets of the moving contactors. The contact states of th contactors are displayed by each piece or overall and electric circuits are driven, whereby the generation of measuring errors is prevented.