Fault gouges were mapped and collected along the Atotsugawa fault, one of the major active faults in Japan, and along the Mozumi–Sukenobu fault, branching off from the Atotsugawa fault. Most of the fault gouge samples contain mica clay minerals, chlorite, smectite, and quartz. To constrain the timing of faulting, K-Ar and x-ray diffraction analyses (XRD) were carried out on mica clay minerals separated from the gouge samples. Each gouge sample was divided into four grain-size fractions of 5–2, 2–1, 1–0.35, and 0.35–0.05 m. Kbler illite crystallinity indices for the finer grain-size fractions (0.05–1 m; illite crystallinity = 0.4–0.8) were found to be higher than those for the coarser fractions (1–5 m; illite crystallinity = 0.3–0.6), indicating the relatively higher concentration of authigenic mica clay minerals. The genesis of the clay minerals was probably related to hydrothermal alteration events associated with fault activity in the finer fractions. K-Ar ages were younger for the finer fractions of all samples. One sample from the Atotsugawa fault yields the youngest age of 61 Ma for the finer two fractions, which probably dates the thermal activity associated with the fault because of a similar age for the finer two fractions. This also suggests that the contamination of protolith mica is negligible for these samples. The finest fraction of the Mozumi–Sukenobu fault gouge derived from the interbedded sandstone and mudstone of the Tetori Group (Upper Jurassic–Lower Cretaceous) gives a K-Ar age of 45 Ma. Although this gouge sample is derived from mudstone and sandstone and, thus, the contamination of protolith illite cannot be identified by illite crystallinity, the age probably approximates that of a hydrothermal alteration event associated with the fault activity, because the age is significantly younger than the sedimentary age of the protolith Tetori Group. These K-Ar ages from the Atotsugawa and Mozumi–Sukenobu faults are compared with previous K-Ar ages of fault gouges from major active faults in Japan, including the Median Tectonic Line in Shikoku–Kinki and the Atera fault. The K-Ar data from these faults indicate that the major active faults in the Inner Zone of Southwest Japan were initiated at 60–50 Ma. Heterogeneity in and around the Late Cretaceous granitic terrane, especially the boundary of rigid granitic body and soft accretionary complex, seems to be the preferred sites for fault initiation.