The Kizawa road tunnel suffered serious damage by the Niigata-Ken Chuetsu earthquake with a magnitude of 6.8 and its damaging aftershocks on October 23, 2004. Authors immediately conducted a series of detailed investigation on its damage, including observation, surveying, measuring width and length of cracks on the tunnel concrete and drawing the cracking map. As a result, it is clearly shown that the cracking on the tunnel concrete can be broadly classified into three patterns; (1) circumferential cracking, (2) circumferential cracking with slightly diagonal angle, and (3) a pair of gigantic longitudinal fissures with an extremely low angle in terms of tunnel axis extending over 35 m. Furthermore, distribution of tensile strain along the tunnel, deformation of tunnel axis in horizontal plane based on integration of difference of opening due to cracks on both side walls and cross sectional deformation due to the gigantic fissures are analyzed from a quantitative point of view, and the results of the analyses and consideration of the features of the active fault-bend region lead discussion on possible mechanism of the three patterns of cracking. Finally, it is concluded that the behavior of mountain with potential landslide likely triggered this damage.