PURPOSE: To secure accuracy in machine operation and to make easy measurement possible by providing a probe in a bore drilled into the outermost layer of a bedrock, and by obtaining modules of deformation for the bedrock by applying load in patterns of intermittence and gradual increase.

CONSTITUTION: A bore 41 is drilled by boring the outermost layer of a bedrock 4, and a probe 2 of an underground load tester is stored therein. Then fluid pressure, led through a high-pressure hose 11 from a pressure source 1, is introduced into a rubber tube 21 that is fitted to the probe 2. With this fluid pressure, load is applied to the peripheral wall of the bore 41 is patterned of intermittence and gradual increase. In this process modulus of deformation DbII for the bedrock expressed in an equation DbII=(1-r)×Rm×(p/r) is obtained based on the load p, displacement r that is displayed on a measureing instrument 3 through a displacement gauge 22, the bore inner radius Rm that is the sum of the inner radius of the bore 41 and the displacement r, and the Poisson's ratio (r) of the bedrock 4.