In the course of field experiments demonstrating engineering feasibility of HDR (Hot Dry Rock) geothermal heat extraction conducted in USA, UK and Japan, low recovery rate of water, which circulates through the heat extraction system consisting of injection and production wellbores and reservoir cracks, has been revealed to be one of the most serious problems against realizing the concept of HDR geothermal heat extraction. Thus it is definitely desirable to be able to estimate the amount of waterloss from the reservoir cracks beforehand prior to hydraulic fracturing for creating the reservoir cracks. The recovery rate of water is directly relating to the in-situ hydraulic properties of the rock mass which is surrounding the reservoir cracks. In the present paper, we propose a new method for measuring insitu hydraulic properties of a rock mass at great depth, more than a thousand meters. The method is based on the analysis of flow of water from the pressurized interval of a straddle packer system, set in a wellbore, back to the sections of wellbore outside the straddle packer system due to permeation through the rock. The method utilizes the time variation of the volumetric flow rate of the back flow stated above for a given level of pressure in the pressurized interval. Since this method can be applied to a variety of levels of the pressure in the pressurized interval, it will be possible to estimate the dependency of hydraulic properties on the reservoir pressure by applying the method proposed in the present paper.