PURPOSE: To eliminate the need of a frequent key operation, to prevent an unselectable state of a window, and to improve the operability and the retrieval function of the window, by providing an active window changing means, and a window changing signal output or stop means.

CONSTITUTION: When a signal from an input device 7 is a start signal or a stop signal, an input control part 6 outputs a start signal or a stop signal to a window changing signal control part 3. The control part 3 starts to output a window changing signal to a window control part 1, by a start signal from the control part 6, and stops the output of the window changing signal, by a stop signal from the control part 6. On the other hand, when the signal from the device 7 is a window operating signal, a control related to the display contents, etc., of an active window is executed. In such a way, plural windows can be made active successively by a simple input operation, therefore, the operability for selecting the active window can be improved, and also, the retrieval function of the window can be improved.