Ground water flow in a surf zone closely relates to the purging of seawater and biological activities. Because the water flow into surf ground is transporting a kind of nutrients and oxygen to benthos in the ground. In the present paper, visualizations of ground water flow show that the water flows into the surf ground with radial direction near the run-up point induced by waves approaching a beach. The seepage water flows out from the seabed surface near the wave breaking point. The measured pressure on the seabed surface reveals the distribution of positive pressure near the run-up point and negative pressure near the wave breaking point, which is the driving force of water percolation in the surf ground. The measured pressure distribution is compared with the calculated by the wave set-up and set-down. Further computation on the ground water flow in the surf ground is conducted assuming Darcy's law. Laplace equation for the time-averaged pressure is solved by finite difference method. The calculated streamlines are in accordance with the visualized flow pattern. It is found that the flow rate of the purging by waves is comparable to that of tidal motion.