It is presumed that the mudflow occurred two times or more in Hakoishi River basin in Mt. Aso. These events happened after the scoria fall-out from Ojo-dake volcano, about 2.7 ka. B. P.. Iron concretions frequently developed in the mudflow deposits and the river bed sediment. In order of iron concretion formation, Takashikozo and nodule, slime grown in the pipe, iron pans were observed in that materials. The iron concretion was formed by the iron-ion produced from weathered scoria in the mudflow deposits, which was molded by the flow of ground water between the mudflow deposit and the river bed sediment. It is possible that this phenomenon influenced upon the slope stability.