To simulate the generation process of tsunami due to a large-scale landslide, the inflow-sediment-induced water-wave generation is investigated by the Lagrangian gridless model of solid-liquid two-phase flow. The physical process of the wave generation can be clarified in detail by tracking the sediment-water-mixing process and the resultant momentum transfer from sediment to water. The MPS method, which discretizes the momentum equation of flow without a numerical diffusion, is applied to track the water-sediment interface and the water-surface wave with very high resolution. The time series of the water-surface elevation are compared with some alternative wave-generating methods, to estimate the wave-making efficiency of inflow sediment.