For a long time, only national statistics offices have had the capability of producing small area statistics on population, such as, in Japan, the Grid Square Statistics of Population Census (GPC). With the proliferation of mobile phones in daily life and the significant development of ICT, there emerges a new kind of statistics called the Mobile Spatial Statistics (MSS), which estimates small area population derived from the operational data of a mobile phone network. There are several differences between the two statistics, particularly relating to measuring de jure versus de facto population, and research was conducted to examine the validity of MSS by comparing the two statistics. Deviation rates were calculated to compare the 2010 GPC population with the MSS population before dawn on the 2010 Census day and concluded that, to a certain degree, MSS would be plausible for use in densely populated areas. We also found that MSS highlighted the busy blocks of central Tokyo early in the morning. The research proved the utility of MSS, and at the same time, it confirmed the ongoing usefulness of GPC as the benchmark of population statistics. © 2013 - IOS Press and the authors. All rights reserved.