The southwestern Pacific Ocean is prone to significant tsunami risk due to giant earthquakes generated near Japan. In this study, two huge tsunamis in the Japan Trench and the Nankai Trough were assumed. They have been called the Sanriku Earthquake Tsunami and the Tonankai-Nankai Earthquake Tsunami. To check the uncertainty of the tsunami model, four more models that intensify the dislocations of half of the tsunami sources were assessed. The computed tsunami energy distributions show that the Sanriku Tsunami propagates mainly toward the central Pacific Ocean and has little effect on the southwestern Pacific countries. On the other hand, the tsunami in the Nankai Trough propagates mainly toward the southwestern Pacific Ocean because of the Mariana Trench. In many cities, the water level exceeds 50 cm. Further, there are some observation stations along the path of the tsunami propagation and they might be useful for a far field tsunami warning system in this region.