Concealed Median Tectonic Line (MTL), the largest fault in Japan, was confirmed in the Kanto Plain. Geologic analyses for core samples obtained from 3500m deep bore hole of the Iwatsuki Observation Well show that MTL runs within 500m south of the lwatsuki well. As MTL partially reactivates as active faults especially in Southwest Japan, the relation between MTL and surface Ayasegawa Active Fault should be clarified, and earthquake disaster prevention should be planned based on the relation.