The effect of surface roughness on the internal fluidity of a water droplet during sliding on an inclined Si surface treated fluoroalkylsilane (FAS) was investigated using particle image velocimetry (PIV). A water droplet slid down by caterpillar-like rotation flow with or without slippage on the solid-liquid interface. The slipping/rolling ratio on the sliding acceleration depended on the surface roughness; the droplet advanced on the rough surface mainly by the rolling mechanism.