PURPOSE: To change a range of a window being an object at a high speed by a simple operation.

CONSTITUTION: Information related to a multi-window function containing window divided position information for showing a position and size in the respective screens of each window, and window discriminating information for showing which of plural windows is an object of preparation/editing of a document, is stored in a window information storage buffer 22. When a prescribed key of a keyboard 35 is depressed and a window zooming function is actuated, a CPU 10 enlarges size of the window being an editing object to the largest size based on information for prescribing the window size set in advance, based on the information in the window information storage buffer 22 in accordance with a multi-window function program in a ROM 11, and also, changes size of other window to a reduced multi-window state.