To perform numerical simulations of tide and tidal current in a semi-closed water area, sea levels or a certain component of tide at open boundaries, which is the sum of incoming waves and outgoing waves through the boundaries, is usually imposed as boundary conditions. It is, however, important to separate incoming and outgoing waves in the research of tidal simulations. Adopting the non-reflecting boundary scheme proposed by Hino, we show that incoming waves into a water area can really be separated from the outgoing waves. Using the scheme, simulations such as the propagation of incident waves from open boundaries in a semi-closed water area are possible. In this way wave propagation characteristics of the area can be identified. Conversely, using the obtained relations between incident waves and resultant surface elevations in the water area, waves incident to the area through the open boundaries can be estimated from the surface displacements observed in the area. This is especially useful if more than one open boundaries exist. This paper reports the process of applying the non-reflection boundary scheme to the tidal calculations and how to estimate incident waves at each open boundary. This process is applied to the tidal calculutions of Osaka Bay. First, incident component of M ₂ tide is estimated from the observed tidal displacements in the bay. Using the estimated incident component, tidal calculations of Osaka Bay is conducted and the results are compared with the observed data. They are also compared with those obtained by a conventional method. Through these comparisons the efficaciousness of the present method is demonstrated.