This paper describes the weathering of the Miocene mudstone in western Shimane Peninsula, from a standpoint of the outcrop occurrence and mineralogy. In this area, the mudstone is classified into three weathering zones (I, II and III zones from the bottom of the outcrops to the top), based on its color and hardness. This classification well agrees with the mineralogical characteristics in each zone : chlorite and pyrite have not been altered in the I zone ; chlorite has partly altered to chlorite-vermiculite interlayer (Ch-V) or vermiculite, and pyrite has partly altered to iron hydroxide (or oxide) in the II zone ; chlorite has mostly altered to Ch-V or vermiculite, and pyrite has mostly altered to iron hydroxide (or oxide) in the III zone. Alteration of chlorite and pyrite changes the color and hardness of mudstone, and plays an important role in forming the weathering zones in this area.