Ultrasonic (seismic) computer tomography (CT) was applied to visualize water infiltration into dry granite and to characterize its presence. The ultrasonic waves carry the information of density, orientation, and aspect ratio of microcracks, and also the information of fluid within them. When the microcracks within a rock are filled with water, the effective bulk modulus of a rock increases, and hence V_p increases. In contrast, V_s is scarcely affected by the presence of fluid. Attention was focused on obtaining the spatial distribution of both V_p and V_s simultaneously. As expected, the V_s maps show no change, although the V_p maps clearly show large changes caused by the permeating water. It is possible to determine the presence of water within microcracks only by simultaneous reconstruction of V_p and V_s.