In order to study a possible response of wide‐open bay, a numerical model of finite difference method was used. For convenience, Tanabe Bay facing the Pacific was taken as a specific example of case study. Response functions of water‐level and currents were obtained numerically at several points in the bay. Water‐level profiles at the crest and trough at the head of the bay show that a resonant mode has a node off the bay mouth on the continental shelf. Current profiles at ebb and flood in the bay were also shown. Such response should be considered for tsunami prediction and protection.