In order to analyze tide level around a narrow‐mouthed bay, a theoretical model was introduced. Two simple models, a square root model and a linearized model, were considered, as the tides in the bay were governed by the tide level difference on the outside and inside of the bay. An analytical solution for the linearized model was studied to provide a basic dynamical understanding. As an example of the application, Osaka Bay in Japan was studied. Tides in the bay at Osaka were estimated by referring to the tides observed at Wakayama, outside of the bay (Problem I). Inversely, estimated tides at Wakayama, referring to the observed tides at Osaka, were studied to find how close the agreement was with the observed tide at Wakayama (Problem II). The linearized model was also applied to the 1960 Chilean Tsunami. By using the model, a short‐range prediction of about one hour may be possible at the intrusion of a tsunami into Osaka Bay. The speculative model seems to give a physical understanding for the lowest mode of oscillations induced in Osaka Bay. A remark to be careful is made when the results are to be used for tsunami prediction.