  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 ISON's 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 ISON's 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.
