An index of submarine volcanic explosivity concerning tsunami generation has been proposed on the basis of the relationship between phreatomagmatic explosion and resultant initial waveform of tsunamis. In the generation process of tsunamis due to phreatomagmatic explosion, the seawater touches magma of high temperature, after which the water evaporates in an instant with explosive increase of volume. We assume the value of this index specifically to simulate tsunamis caused by submarine volcanic eruption in Kagoshima Bay, where submarine explosion has been observed. Furthermore, tsunamis due to subsidence of a caldera near a volcanic mountain in the same bay are numerically simulated using an initial tsunami profile assumed without discussion about the emission process of magma out of a chamber through submarine volcanic eruption.