Pore water pressure and liquefaction in a saturated sand bed induced by tsunami are theoretically and experimentally discussed. A governing equation on the basis of the Darcy's law is presented, and a centrifugal model test is carried out to verify the applicability of the governing equation. In addition, numerical simulations are performed in one-dimension, giving observed sea levels by GPS buoys on 11 March, 2011 (MLIT Japan) as the input data, in order to evaluate pore pressure and liquefaction by the actual tsunami. Moreover, parametric studies are performed assuming a permeable layer as silt, and the influence of permeability on liquefaction is discussed. Experimental results show that the effective stress decreases due to tsunami. It is evident that the calculated values are in good agreement with the observed values, and the governing equation is applicable to evaluate the liquefaction potential. In addition, it is revealed that liquefaction can occur due to the actual tsunami even if the permeable layer is comparatively good drainage like sandy soil. It is also clear that the excess pore water pressure ratio increases with the decrease of permeability at the shallow depth, and the influence ranges up to the depth of 4 m below the ground.