We report the electron density in a plasma tail of Comet ISON (C/2012 S1) derived from interplanetary scintillation (IPS) observations during November 1--28, 2013. Comet ISON showed a well-developed plasma tail (longer than 2.98 x 10^{7} km) before its perihelion passage on November 28. We identified a radio source whose line-of-sight approached the ISONs plasma tail in the above period and obtained its IPS data using the Solar Wind Imaging Facility at 327 MHz. We used the Heliospheric Imager onboard the Solar-Terrestrial Relation Observatory to distinguish between the cometary tail and solar eruption origins of their enhanced scintillation. From our examinations, we confirmed three IPS enhancements of a radio source 1148-00 on November 13, 16, and 17, which could be attributed to the disturbance in the cometary tail. Power spectra of 1148-00 had the steeper slope than normal ones during its occultation by the plasma tail. We estimated the electron density in the ISONs plasma tail and found 84 cm^{-3} around the tail axis at a distance of 3.74 x 10^{7} km from the cometary nucleus and an unexpected variation of the electron density in the vicinity of the tail boundary.
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