The tsunami generated by the 1998 Papua New Guinea earthquake was observed at several tide gauges and ocean bottom pressure gauges in Japan. The fault model of the 1998 Papua New Guinea earthquake was estimated using those tsunami waveforms observed in Japan and coseismic subsidence observed along the coast of Papua New Guinea near the source region. The numerical simulation of the tsunami using the linear Boussinesq equation was carried out. The tsunami waveforms in Japan are explained by the fault model which is consistent with the Harvard CMT solution including the seismic moment of 3.7 × 1019 Nm. The additional tsunami source, such as a sediment slump, is not needed to explain the observed tsunami in Japan. The mechanism of the earthquake was likely to be a steeply dipping reverse fault. The 1998 Papua New Guinea earthquake was likely to occur within a small accretionary prism near the New Guinea trench.