A non-magnetic mass spectrometer of path separation type with parallel plane electrodes was studied. Its sensor consists of two side-plates and a rectangular box inserted between them. The box electrode on which a negative r. f. voltage of square wave is applied, has two windows with grids on both sides. A collimated ion beam injected into the space between the box and the first side-plate along the tube axis is accelerated to the box and then decelerated between the box and the other side-plate after passed through the box. The composite ions in the beam disperse in direction corresponding to mass number and only ions directed along the tube axis can reach the collector through the exit collimator. In order to obtain mass spectra, the frequency of r. f. voltage was swept. It is found that the resolving power of the spectrometer studied here is 27 and the current efficiency is about 4% which is in good agreement with the expected values. The resolving power does not depend on the velocity distribution of ions. Theoretically obtainable efficiency would be 50%. A method to improve the resolving power was tested. Further projects are proposed to improve resolution and sensitivity.