PURPOSE: To simplify the enlarging/reducing operation of a size of a window by moving a pointer by setting a tiling pattern related to plural windows, and interlocking and changing a position and a size of each window.

CONSTITUTION: For instance, a position of a pointer P is moved as a pointer P' shown in the figure by a mouse, etc., under a state that a display of three pieces of windows is being executed. In this case, a window managing function part 1 knows a position of the pointer P', and calculates a position and a size of each window (a), (b) and (c) such as the pointer P' becomes a boundary under a given tiling pattern. Subsequently, the window managing function part 1 informs a processing program corresponding to each window, and a display corresponding to the respective new window sizes is executed. In such a way, since it is sufficient by only moving the position of the pointer P, the operation for enlarging and reducing the size is simplified.