After an operator has roughly moved a plurality of windows 101-103, with a graph (e.g. spectrum) placed in each window, to desired positions by a manual operation, initial positional coordinates of each window are acquired. The operator also makes an input of the selection of an arrangement pattern (e.g. vertical arrangement). According to the selected arrangement pattern and the number of windows, the sizes of display areas to be formed by dividing a main display frame 100 are calculated, and the display position of each window is also determined. Then, the display order of the windows is calculated from the relative positional relationship of the windows revealed from their initial positional coordinates, and a display screen is eventually formed by resizing each window so as to fit it to the size of the display area and then arranging the windows in the calculated display order.