Surface runoff is likely to occur on ashfall deposit slopes due to a decrease in the infiltration capacity of the ground surface. Although the mechanism of reduced infiltration capacity due to increasing fine particle content (Fc) in volcanic ash has been clarified, the effect of Fc on surface runoff processes on slopes with thin deposits of ashfall is not well understood. Therefore, we conducted a laboratory rainfall simulation experiment (simulated intensities of 5 mm h^-1,15mmh^-1, and 30 mm h^-1) on slopes with 1 cm thick deposits of volcanic ash with different Fc. Experimental plots were prepared by depositing a volcanic ash layer on highly permeable silica sand. Runoff water and sediment from the experimental plots did not occur at 5 mm h^-1 regardless of Fc, while at 15 mm h^-1, they did and did not occur at the same Fc. On the other hand, runoff water and sediment tended to increase with increasing Fc at 30 mm h^-1. Sediment yield tended to increase with an increasing amount of runoff water. The estimated maximum infiltration rate (α ) of the experimental plots was higher than the given simulated intensity and tended to decrease with increasing Fc. These results suggest that surface runoff may occur on slopes with thin deposits of ashfall, even under conditions where the rainfall intensity is below the maximum infiltration rate.