Na ion distribution in the one-dimensional tunnel of priderite has been studied by the single-crystal X-ray diffraction technique. Electron-density maps drawn by the maximum-entropy method (MEM) have clearly demonstrated a broad and continuous distribution. Na ions are partly concentrated at four equivalent positions deviated from the tunnel axis in a large cavity. The center of the oxygens-square connecting the cavities are also available for the Na site. Another peak can be found on the tunnel axis, which are ca. 0.45 Angstrom away from the above position. These are remarkable characteristics of Na-priderite which have never been observed in K-priderites, the well-known one-dimensional ion conductors.