Crack depth indicator using a direct current potential drop technique has been generally applied to the nondestructive inspection in the thermal power plants such as steam pipe and turbine casing. In this paper, the authors presented practical studies concerning the development of the advanced crack depth indicator for the deep surface crack. The authors have conducted experimental measurements using the customary crack depth indicator inspecting the fatigue pre-cracked plate specimens of some materials. The results showed that the crack depth indicator was effective to inspect the surface shallow crack, however, it was hardly applied to the deep crack. And the sensor of this equipment was found to be difficult to apply some practical conditions such as contact surfaces and mechanical designs of inspected structures. To get over the restrictions, the authors have developed a new type of crack depth indicator and sensor. The authors have conducted additional experimental measurements applying this equipment using some pre-cracked specimens such as plates and structural components of some materials.