The humidity-dependent acoustic emission (AE) activity in a quartz gouge layer was investigated via sliding-rate step tests. Because AE events are generated by the brittle failure of the contact junction, their activity reflects the micromechanics of friction. AE activity was evaluated by the m-value (characterizing amplitude distribution), AE rate (number of events per unit sliding distance), and their sliding-rate dependences. The m-value decreased with increasing humidity, suggesting contact junction growth by the pressure solution. Increased humidity decreased the AE rate and enhanced the rate-weakening of friction, implying the role of water in suppressing the brittleness of contact junction and strengthening macroscopic instability. Notably, the relationship between the direct effect of friction and the sliding-rate dependence of AE rate under dry conditions was distinct from those under other conditions, suggesting these are sensitivity to areal fraction of the water film on the surface of the gouge particle.