Shake table tests on levees deteriorated by seepage-induced internal erosion are performed in a geotechnical centrifuge to investigate the effects of erosion-induced heterogeneity and stiffness/strength reduction on the seismic response of the levees. In the experiments, a model embankment made of gap-graded soil is first subjected to repeated seepage flow by changing the water level in the flood channel. After lowering the water level, earthquake motions are applied to the model deteriorated by seepage-induced internal erosion, subsequently. The test results reveal that the natural frequency and equivalent shear wave velocity of the embankment significantly decrease with the seepage-induced internal erosion, suggesting that reduction of the soil stiffness occurs due to the internal erosion.