Biomass utilization is one of the most important methods for global warming mitigation from the viewpoints of both carbon sequestration and carbon-neutral energy use. Carbon sequestration potential should result from phytomass stock expansion, and in this study, the potential of phytomass stock expansion due to afforestation/rehabilitation with additional special water management was estimated by using several sets of global geographical information systems (GIS) data. The estimation was conducted under unified conditions for all the world areas by using a computer program based on the GIS data. The carbon sequestration potential of a land area was assumed to be primarily determined by its precipitation level if the area satisfied certain meteorological and geographical conditions for phytomass stock expansion, such as land use, temperature, slope, and soil type, and the sensitivities of these conditions were studied. The carbon sequestration potential due to regular afforestation/rehabilitation was estimated to be about 169 GtC in the world, and the effective regions for carbon sequestration were Brazil, etc. On the other hand, the increase in the carbon sequestration potential due to the additional special water management was estimated to be about 21 GtC in the world, and the effective regions were Saudi Arabia, west Australia, etc.