This study experimentally simulated dehydration- and dehydroxylation-induced weakening of clay-rich gouges to investigate the potential of phyllosilicate minerals to generate abnormal pore pressure, triggering earthquakes. For the experiment, the gouge samples were subjected to sliding with heating at 9.6°C/min to 500°C under 80 MPa of confining pressure. In the undrained condition in which expulsed water was confined in the gouges, significant frictional strength reductions were found in the Na-montmorillonite gouge due to dehydration and in the kaolinite gouge due to dehydroxylation. However, frictional strength continued to increase in the fully drained condition that allowed released water to move outside the gouge. Frictional strengths under the two drained conditions were compared during dehydration and dehydroxylation reactions to estimate the pore pressure generated in the gouge zone under the undrained condition. The 6∼8 wt % of dehydroxylated water generated from the kaolinite caused a rapid increase in pore pressure to 68 MPa at 500°C. In contrast, in the Na-montmorillonite gouge, the pore pressure gradually increased to 72 MPa as the temperature rose to 500°C. Especially on a coseismic stage, these results suggest that dehydration/dehydroxylation could advance the frictional sliding acceleration induced by frictional heating, even if the fault is in a dry condition.