The paper describes a 3-dimensional numerical simulation technique based on the Marker-And-Cell (MAC) method. In order to simulate sloshing motion of liquid in a tank with such 3-dimensional shapes as shamfers, the governing equations are derived in a tank wall fitted coordinates system which moves with the tank. Comparing with the case where cartesian coordinates system is used, the boundary condition on the tank walls can be expressed simply in a numerical procedure, and the effects of inclined walls of the tank on slosihng motion can be taken into account with reasonable numbers of computational cells. The 3-dimensional configuration of a free surface caused by chamfers are observed experimentally, though the tank model is forced to oscillate in only longitudinal direction. Numerical results succeed to show the above tendency. And also, the calculated time histories of sloshing pressure on the tank wall agree well with the experimental ones.