We analyzed temporal changes in the quality factor (Q) of P waves transmitting through a fracturing Westerly granite sample under triaxial compressive conditions, using the rise time of the initial wave. Broadband recording was performed throughout, even after the differential stress reached the peak strength. We calibrated a transducer assembly and corrected the recorded waveforms. Using theoretical waveforms, we derived a new equation relating QP, the rise time for the velocity waveform, and the travel time. QP remained at ∼23 before the start of dilatancy and decreased to ∼17 after the peak stress was reached. A large nonlinear decrease in QP was not clearly observed, while the first wave amplitude clearly decreased rapidly after the occurrence of the peak stress.