This paper reports the results of experimental investigation into the erosion and failure of the volcanic ash slopes. The major results obtained were: 1) The initial infiltration capacity of the volcanic ash layer is about 6×10^<-2>cm/sec, while the final capacity is between 5×10^<-4>cm/sec and 1×10<-2>cm/sec. 2) With rainfall patterns which increase the water content gully erosion develops from the top of the slope. 3) High permeability of the lower parts of the slope contributes greatly to its stability. 4) Failure usually occurs when pore water pressure attains a constant value. 5) The conditions under which failure doesn't occur even though rainfall intensity exceeds infiltration capacity and causes runoff.