The mechanism of the water exchange by a vertical wall with a small aperture installed at the middle of the water depth is investigated experimentally. Two different physical model experiments were performed using visualization techniques. The laser induced fluorescence (LIF) were performed to observe mixing of bulk water between inner and outer ocean through the slit. On the basis of LIF experiments, the time-averaged velocity near the slit were measured by the particle image velocimetry (PIV) method. The mixing of bulk water by the slit induced current was significant for longer wave period. The influence area of water exchange by the slit was approximately 1/3 of incident wave length. © 2007 World Scientific Publishing Co. Pte. Ltd.