PURPOSE: To determine the position of a central electrode along the axis accurately by applying an AC input voltage to a tubular intermediate electrode and determining the difference between signals appearing on two tubular end electrodes.

CONSTITUTION: A good AC input voltage produces a signal appearing at a central electrode even if it is a normal sine wave voltage. That signal causes to produce signals on one and the other tubular electrodes 1, 3 sequentially. These signals are determined by signals C1, C2 at terminal 7 and the signal at terminal 8 is determined by signals C1, C2. Assuming the central electrode is confined within the region of an intermediate tubular electrode 2, capacitance of a capacitor formed by the central electrode and intermediate tubular electrode is invariant and thereby the C1 is kept constant. When the central electrode moves along an axis 5, physical properties of a capacitor formed by two outer electrodes 1, 3 and the central electrode are varied to cause variation of C2, C3. Consequently, the signals appearing at the terminals 7, 8 provide a positional reference of central electrode already.