Although self-excited sloshing phenomena of free surface are sometimes observed in an outlet pit of electric power plants, the onset mechanisms are not clearly understood. Numerical analysis based on two dimensional geometry was conducted to evaluate such phenomena using moving grid transformation method. In the analyses, third order finite difference scheme is used for discretization of convective terms in momentum equations describing flow behavior, while boundary conditions at moving free surface are completely modeled considering surface tension and shear forces. Finally, it was shown that the analysis can simulate the self-excited phenomena due to internal flow depending on Froude and Reynolds numbers.