Water-level difference induced by tsunami generates one-way seepage flow in a breakwater mound. It has been pointed out that seepage flow decreases bearing capacity of a mound. The flow may also trigger piping and/or boiling of a mound, and scouring of a sand foundation. The present study describes the stability of a breakwater foundation under seepage flow, by using the techniques of a centrifuge model test and a FEM analysis. Experimental results showed how piping and boiling occurred and that seepage flow decreased the bearing capacity of a mound. Results of model tests and numerical analyses also illustrated the effect of reduction of bearing capacity under seepage.