Joints are markedly developed in the acidic welded tuff of the Sandankyo Area, Hiroshima Prefecture, Southwest Japan. Six sets of joint are distinguished in this area, namely N-S set, NESW set, NNE-SSW set, NW-SE set, NNW-SSE set, and horizontal set. It is considered that NESW, NNE-SSW, NW-SE, and NNW-SSE sets of joint were formed by the simple shear due to NE-SW strike-slip faulting with right-lateral shear sense. Detailed microscopy shows that N-S and NE-SW sets of joint contain narrow quartz vein with newly recrystallized muscovite, and cut by the other sets. This suggests that, preceding the NE-SW faulting, WNW-ESE extention accompanied with granite porphyry intrusion formed N-S and NE-SW sets of joint.