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Electron densities in a coaxial plasma opening switch were measured at many lines-of-sight. In the present experiment, electron density was measured by a He-Ne laser interferometer with chordal lines-of-sight. In order to observe the motion of the plasma opening switch (POS) plasma, the electron density contours during the conduction, opening, and postopening phases were drawn by combining the results of interferometer experiments. The radial and axial motion of POS plasma was investigated from the density contours. As conduction time progressed, the POS plasma moved toward downstream. At 800 ns, which corresponds to the time of opening in the current waveform, low-density region less than 1015 cm-2 is seen at 10 mm from the cathode. After the opening was completed, the low-density gap disappeared and the shape of the corn-shape-like plasma was distorted. The interferometer experiment was followed by the additional spectroscopic experiment. The results of spectroscopic experiment were in good agreement with those of interferometer experiment.