Defined information is generated for each of screen information, layout information, and media information in response to an operator's entry when an environment is defined. When a change is monitored in predetermined position information using a screen monitoring unit during the actual operation, a character string to be displayed with media information is extracted according to the contents of the defined information. After media information corresponding to the character string, that is, the name of layout to be used and actual media information is obtained, and the position and size of a frame and the display format of the media are further obtained, each depending on the name of layout to be used, media information corresponding to the character string to be displayed with media information is displayed. Thus, character string information can be displayed with multimedia information added together only by mounting the multimedia information add-on system without restructuring an existing system.