a device for measuring the amount of movement of a straightly moving object is provided with a sensor unit made up of the straightly moving object and stationary members arranged to form two pairs of capacitors therebetween . the confronting surfaces of the moving object and stationary members are graduated with gear - like cuts so that while the capacitances of two of the capacitors are maximum , the capacitance of the other capacitors are minimum . the device further comprises a pair of oscillators coupled to the two pairs of capacitors , and a mixer connected to the oscillators so that the difference in capacitance between the capacitors is obtained as the difference in frequency , which is processed to provide the amount of movement of the moving object .