|We, Dam Working Group of Research Group of Engineering Geology in JSEG, have an activity for the purpose of clarifying appropriate method and density of geological survey by geological type. So far, we have carried out an overlay analysis by “Geographic Information System” in order to quantify the concordance rate of geological and rock mass classification maps for foundation rocks of gravity type concrete dam between investigation stage and excavation stage. Based on the qualified concordance rate, relationship between the concordance rate and geological survey density, and characteristics of concordance rate by geological type haves been examined. Total number of dams examined until now is 18. Type of geology at dam site is normal sediments (3 dams), accretionary deposits (4 dams), volcanic rocks (5 dams), plutonic rocks (5 dams) and metamorphic rocks (1 dam). The concordance rate is defined as the ratio of the area where the distribution of geology and rock mass classification by the excavation result matches estimation by result of geological survey to the total area of the dam foundation surface. Regarding the rock mass classification map, the ratio of area where the rock mass classification of the excavation result is higher than the estimation by the result of survey is defined as Rate (S), and in case that the excavation result is lower than the result of survey is defined as Rate (R), and the ratio of area where the excavation result matches the result of survey is defined as "Perfect Concordance Rate (P)". Also the sum of P & S is defined as “Safety Side Concordance Rate (P+S)”. Survey density is defined as the total surface area of investigated boreholes and wall area of adits carried out per area of 100m2. In this report, the relationship between concordance rate of geological maps and survey density, and the relationship between Perfect Concordance Rate (P) and Safety Side Concordance Rate (P+S) of rock mass classification map by geological type, and engineering geological factor of dams with low concordance rate are reported.