In this paper, we consider an automatic warehouse, in which rc-k palettes are arranged two-dimensionally on a grid with r rows c columns. The palettes in the warehouse can slide independently and simultaneously with the use of k spaces, unless they come into collision. Then, we show the minimum number of sliding operations required to move a specified palette to a specified position, where one sliding operation is a set of slides of palettes which are executed simultaneously in a unit time withouto any collision.