A non-contact method of determining displacement of a rigid plate facing the open end of an acoustic resonance tube by measuring the amplitude of acoustic pressure inside the tube with constant excitation (or alternately, variation of the resonance frequency) is described. The method features itself by being independent of electric/magnetic properties of the object, not requiring compressed air supply, and not excerting any force on the object. By suitable choice for the excitation frequency and position of sound pressure pick up, rms pressure amplitude can be made change almost linearly with the displacement, by a factor up to 10 For a 2-dimensional case, both a finite difference calculation and experiment indicated good prospect for such method. The validity of the calculation was confirmed by agreement of its results with the experimental. By numerical simulation based on the calculation, linearity of output-displacement relation, magnitude of fractional change in the output, susceptibility of the output to pick-up position change, and relative level of sound pressure for constant frequencies and pick-up positions. Results are summarized in a map of the above-named characteristics with frequency and pick-up position as the coordinates. It is seen that a frequency a little lower than the fundamental (or the 3rd harmonic) resonance frequency of a tube with one end closed and the other end open gives satisfactory condition.