In order to develop the on-line measurement system of sliver linear density-variation, the authors made, by try-and-error method, a pneumatic micrometer device which is consisted of specially designed trumpet for passing the sliver-specimen and a back pressure tank with strain transducer. When several kinds of sliver were passed through the trumpet, the back pressure variation in the pressure tank were sensed. And the back pressure-variation data(experiment data) were recorded by X-Y plotter. The experimental data were compared with the data from Uster Evenness Tester for the same sliver specimen. It was founded that, by this comparison, the back pressure-variation data measured by this experiment were proportional to the sliver linear density variation only if the average value of back pressure-variations were proportional to the weight of sliver. The slight deviations between the data from Uster Evenness Tester and from this experiment are caused by the structural characteristics of constituent fibers in sliver specimen.