Hydraulic fracturing is applied for oil and gas production enhancement not only to hard rocks but also to soft rocks such as unconsolidated sands. In the latter case, fracturing behavior is not suitably represented by existing models for brittle, linear-elastic rocks. Thus we developed an apparatus for carrying out hydraulic fracturing experiment in a cylindrical specimen under triaxial compression and tried to observe the dynamic behavior of hydraulically-induced fracture in unconsolidated sands by using a x-ray CT scanner. In those CT images, we could detect clearly how the fracture propagated and then the fracturing fluid infiltrated around the fracture. As a result, we found that the shapes of the infiltrated regions changed obviously for two cases when the fracture was and was not induced, while there was a small difference in the peak pressure of fracturing fluid. Those phenomena indicate that permeability of the specimen play an important role rather than fluid pressure on fracture formation.