PURPOSE: To enable stable measurement with an up-down counter by housing an electrode part into a shielding case to prevent said part from being affected by temp. and converting immediately the change in the capacity between electrodes into a pulse.

CONSTITUTION: A slider 4 is moved along a beam 1. A rack 3 equipped to a beam 1 and pinions 121 and 122 supported freely rotatably to a shielding case 10 are engaged with each other and therefore the pinions move and a center pinion 14 and a shaft 142 are also turned by over-drive gears 131 and 132. When a main electrode 15 fixed to the shaft 142 moves cooperatively with said shaft, a continuous signal appears at the terminals 164 and 165 of a detecting electrode 16 facing said main electrode apart at a space. The phase difference from a reference signal is detected by a comparator and the output is inputted to an up- down counter and is digitally displayed on a display device. A sensor is housed in the case 10 in the above-mentioned way and the change in the capacity between the electrodes is immediately converted to a pulse and is detected, by which the stable measurement is accomplished.