A program executing machine generates display image data including data of at least first and second image areas, and changes a display parameter of the second image area in accordance with a characteristic parameter detected in the first image area. The characteristic parameter is, e.g., the mean brightness value of a predetermined area in a dynamic image within the first image area. The brightness of the entire or part of the second image area is changed in accordance with the mean brightness value of the predetermined area. After detecting the mean brightness value of the predetermined area as the characteristic parameter, the program executing machine changes the brightness of a predetermined area within the second image area. A video game with higher reality and higher entertainment is thus realized.