Despite the great importance of slip weakening during earthquakes, the fault-weakening mechanism inside the fault remains to be clarified. Here we propose a micromechanism of slip weakening at seismic and subseismic slip rates as demonstrated by electrical conductivity observation across the fault. At the seismic slip rate, the formation of melt patches and the subsequent growth to molten layer during frictional melting were confirmed by the rapid increases in conductivity in two stages. At the subseismic slip rate, the conductivity data successfully captured rapid asperity breakage phases and the subsequent gouge evolution phases during the slip-weakening process. The present results suggest that the gouge comminution process is one of the key mechanisms controlling the slip stability at the subseismic slip rate, which may cause a variety of natural earthquakes.