To enable estimation of ground strength without practical investigation and enhance the reliability of the investigation results by estimating the round strength on the basis of the ground condition of a topography.

The ground strength of each ground condition is estimated using a ground condition data base showing the relation with the ground condition and estimation ground strength stored to a memory 4. The location of a construction execution predetermined site, more particularly an address is inputted by an input device 1, strength estimation processing is performed by a controller 2, the ground strength of the building execution predetermined site is estimated, and stored to a memory 4 as the ground strength data bas classified by the building execution predetermined site. Next, an investigation result which a worker practically performs is inputted, and stored to the ground strength data base in the memory 4. The ground strength investigation result obtained in the above and the estimation ground strength are compared with each other by the controller 2, and reliability of the investigation result is assessed. If the estimation ground strength and the investigation result are the same ground strength, the investigation result is stored to the ground strength data base as the determined ground strength of the building execution predetermined site, if they are different from each other, reinvestigation is performed, and the investigation result is stored priorly.