Hydrophobicity and sliding behavior of a water drop are very important phenomena in our daily life as well as in many industrial processes. However, the difference between these two phenomena is not well understood. This study was performed to investigate the movement of the water drops on the inclined surfaces of the silicone films and was focused on the friction force between the water drops and the inclined surfaces. Using a high-speed camera system to observe the sliding movement of the water drops, the sliding speed of the water drops under the various tilt angles and the various drop volumes was investigated. The sliding speed of the water drops increased with increasing the tilt angle and the drop volume. Equation of motion was used to calculate the friction force from the sliding movement. The friction force was proportional to the power of the terminal sliding speed. It was clarified that the main cause of the friction force was the surface tension, and the secondary cause was the viscous resistance. This result is expected to contribute the prediction of the sliding behavior of water drops.