Here we test the precursory enhancement in ionospheric total electron content (TEC) measured by GNSS leading up to the 2011 Mw9.0 Tohoku-Oki Earthquake. We verify the frequency of this TEC enhancement via analysis of a two-month vertical TEC (VTEC) time series that included the Tohoku-Oki Earthquake using the procedure, based on Akaike’s information criterion, and threshold of Heki and Enomoto [2015]. The averaged occurrence rate of the TEC enhancement is much larger than that reported by Heki and Enomoto [2015] when all of the visible GPS satellites at a given station are taken into account. We cannot rule out the possibility that the pre-seismic VTEC changes before the great earthquakes that were reported by Heki and Enomoto [2015] are just a product of chance. Furthermore, we analyze the spatial distribution of the pre-seismic TEC enhancement and co-seismic TEC depletion for the Tohoku-Oki Earthquake. We observe significant post-seismic depletion that lasted at least 2 h after the earthquake and extended at least 500 km from the center of the large-slip area. The spatial distribution of this post-seismic depletion appears to be independent of the pre-seismic enhancement. The enhancement reported by Heki [2011] before the Tohoku-Oki Earthquake may therefore be an apparent phenomenon related to the combined effects of a largescale traveling ionosphere disturbance and co-seismic ionospheric disturbance.