It has been clarified that sloshing motions in ship tanks can be estimated by numerical calculations such as MAC method. But, even now, it is not easy to evaluate the magnitude of impact pressures. In this study, to clarify the characteristic of calculated impact pressures, the mechanism how impact pressures are obtained in the numerical procedure is investigated as one dimensional problem. And, impact pressures obtained from 3 dimensional calculations are compared with above results. As a result, the following characteristics are clarified. 1) When incompressibility is assumed, the magnitude of calculated impact pressures is in inverse proportion to numerical time step, but the impulse is unchanged. 2) When pseudocompressibility is introduced, the magnitude of calculated impact pressures converges to a constant value in case very small numerical time step is used.