We were compared classification of the rock mass including open cracks (opening width is 0.5 - 2 mm) by observing open cracks in the borehole and in the inspection adit, and by using seismic tomography. As a result, it was confirmed that can be divided by the seismic velocity(V_p) quantitatively into the zone of including open cracks and including almost no open cracks. The seismic velocity of the zone including open cracks was 0.5-2.4 km/s, the seismic velocity of the zone including almost no open cracks was 2.4-4.0 km/s.