PURPOSE: To exactly measure a value N, by a method wherein an inground penetration search cone is moved down in a rod after an inground penetration rod is inserted into soil, the cone is protruded from the lower end of the rod, to be fixed, and the rod is moved down.

CONSTITUTION: A machine frame set on a jack-up bearer is provided with a groud-excavation-rod-penetraiung device 7, and the penetrating device 7 is worked, and an excavation slot 2 is formed by a ground excavation rod 3. Then, after a ground-penetrating cone 4 is moved down in the rod 3 with a cable 5 and is protruded from the lower end of the rod 3, the telescoping device 16 of the cone 4 is engaged with a fitting groove 22 on the internal surface of the rod 3. In this state, the rod 3 is moved down to press the cone 4 into ground to measure a value N. As a result, the value N can be measured exactly without being influenced by a hardened layer in poor subsoil.