The non-dimensional bed shear stress in Osaka Bay by a tsunami caused by the Nankai Trough Earthquake was calculated, and the conditions under which the resuspension of marine sediments would occur were determined in every region of Osaka Bay. Resuspension does not occur in the western part of Osaka Bay, where the predicted maximum flow velocity of the tsunami is 1.8 m/s, under the present condition of the water depth and the moisture content. On the other hand, resuspension does occur in the eastern part of Osaka Bay by the predicted tsunami. The occurrence and the scale of the resuspension in the eastern part depend on the moisture content and the flow velocity of the tsunami. An extremely strong resuspension will occur continuously throughout a tsunami hitting the estuary with over 72 % moisture content. The scale of the resuspension in such a "hot spot" is determined by the moisture content. Marine sediments contain cysts and heavy metals, especially in the estuary. We are afraid that resuspension of the sediment may lead to red tide and water pollution over a wide area due to advection after the tsunami.