To investigate how the constitutive parameters prescribing the slip-dependent law for the shear failure of intact rock are affected by strain rate, we conducted a series of triaxial fracture experiments at a strain rate in the range from 10-5 to 10-7/s in seismogenic environments using Tsukuba granite samples. The experimental results showed that the peak shear strength decreases modestly with a reduction in strain rate, according to a logarithmic law. The reduction rate tested at wet conditions was larger than at dry conditions. It was found that the maximum slip-weakening rate slightly increases with an increase in strain rate at wet conditions. This indicates that the mechanical instability of the shear failure process is slightly enhanced by an increase in strain rate.