Demand of water to maintain stomata open enough to get carbon dioxide from ambient atmosphere during daytime may be determined solely by the above ground conditions as climate conditions and plant's size and shape, whereas possible amount of water that the plant absorbs from the soil may be determined solely by soil-water conditions, rooting system of the plant and the force to absorb water from the soil through the plant. We want to know ways to calculate the Water-Absorption Rate (WAR) of the plant. A rooting system of the plant may consist of size and volume of water-absorbing plant root. The soil-water conditions may consist of characteristics of retention and conductivity of the soil water, and the soil wetness in the site. We considered numbers of a single root system. Each single root system has a cylindrical root which radius is a [m] and length is L [m] placed at a center of a cylindrical soil-root system which radius is b [m] and length L [m]. Partial differential equation to express this situation is hard to solve because of the non-linear characteristics of the non-linearity of the unsaturated hydraulic conductivity of soils. So, we have considered a system at a so-called quasi steady state, where dT/dt is independent of position r [m] but a constant. Assuming quasi-steady conditions, the partial differential equation is solved analytically. The solution that expresses WAR of a soil-plant system has shown as a product of the root length, L, a simple function of a ratio of root volume to the volume of the soil-root system, v [ m³/m³ ], and a difference of matric flux potential ψ for the soil wetness θ at r=b and that of at r=a. Obtaining this solution, we tried to corfirm it with measurement using tomato and eggplant. Our results have not been completely satisfactory but need more tests. We desire that thus obtained new formula to be well known and they are tested in various conditions in the world to get better satisfaction.