Precise prediction of water level in a river mouth is of practical importance for flood inundation modeling in the vicinity of a river mouth. The water level in a river entrance is influenced by several external forces, such as river discharge, tidal motion and wave action, though, very little attention has been paid to the effect of waves. In recent years, several wave set-up observations at a river mouth have been reported. From those results, it has been concluded that the height of wave set-up shows distinct dependence on the geomorphology of river entrance. However, quantitative relationship between the magnitude of wave set-up height and wave condition has not thoroughly been clarified yet. In this study, a method is proposed for estimating time-dependent wave set-up height at a river entrance, and the result is compared with laboratory data by the present authors.