The possibility of the method of measuring root hardening diameter was investigated by using resistivity. In this study, laboratory experiments and in-site experiments were conducted. The electrode arrangement is a 4-pole method. As a laboratory experiment, we investigated the relationships between the resistivity and the boundary position of the formation, exploration depth. Based on the results, measuring method using resistivity for in-site was developed. And it was conformed that the resistivity was extremely effective to measure the diameter of foot protection in the site.