The Boundary Element Method based on perturbation is applied to sloshing analysis. In usual sloshing analysis by the Boundary Element Method, a flow field is solved at each time step, assembling the matrix corresponding to the boundary integral equations for the deformed domain. At this time, if an analysis for the deformed field is carried out approximately with the Boundary Element Method based on perturbation, it is not necessary to re-assemble equations and therefore the calculation time would be decreased. For example, the calculation time is decreased to 1/280 in a problem which has 40 D. O. F.. The reduction ratio of the calculation time improves with an increase in the number of D. O. F..