The principle of determining linear displacement of a (quasy) rigid plane facing to the only open end of a duct through measurement of acoustic pressure in the duct, reported previously by the author, was extended to the axi-symmetric configuration. A simple calculation procedure was confirmed to give reliable results except for near-resonance amplitude, through conparison with experimental results. Operational characteristics such as linearity of acoustic pressure, extent of its variation, sensitivity of output to pick up position, and magnitude of response to the acoustic excitation were determined by numerical simulation based on the calculation, and results were summarised into a map of characteristics as functions of frequency and pick up position. Excellent linearity was predicted for (and only for) frequencies about 95% and 285% of the fundamental resonance frequency of the tube with one end open and the other closed. Experimentally, however, excellent linearity could be achieved for the gap length only up to half the radius of the tube, the limit being about a half of that for the 2-dimensional case. This difference is not surprising in view of the increased importance of peripheral cross section of the gap in relation to the axial cross-section of the tube in the axi-symmetric case. Hence, the operational range of the gap (plane displacement) was limited to 0 to half the radius. For this re-defined range, linearity of the output etc. were reexamined, results showing that fairly good characteristics can be obtained for rather wide range of conditions around those frequencies given above. Determination of displacement through resonance frequency variation was also referred to.