The present invention provides a technique to improve productivity on a task for redesigning a screen depending on change in a display size of the screen displayed on a display unit. In a computer environment, a display of a screen composed of a container for defining a frame of the screen and a GUI part arranged in this container is controlled as follows. When an event is detected for changing the size of the screen displayed on a display section, the size of the container in this screen is changed according to the detected event. Furthermore, data relating to a GUI part arranged within the container are changed on the basis of transformation assist lines 341 and 342 set on the container. Then the changed size of the container, a position and the size of the GUI part are reflected to update the screen.