Tsunamis generated by a voluminous pyroclastic flow entering the sea during a caldera-forming eruption at the Kikai caldera, Japan, were investigated by using a two-layer shallow water model which is limited to the source conditions and their impact on coastal areas. Volume flux of the dense component of the pyroclastic flow was controlled by a sine function. Results showed that the maximum height of the tsunami was largest in models with the largest volume flux of the flows. The approximate source conditions of the tsunami, which can stir sediment particles on the sea floor, were investigated using the non-dimensional boundary shear stress. Results from our simulation showed that the shear stress to initiate movement of sediment particles was not easily achieved in areas where there was evidence of a tsunami. A caldera collapse is thought more likely to have generated the huge tsunami rather than a pyroclastic flow.