An objective is to effectively specify a method of synchronizing a video image with computer processing. At step 1200, a frame in which a script is desired to be defined is selected and displayed on the editing region 200 of a display screen. Then, at step 1210, when an item "script" is selected from the editing menu 130, the script editor is opened. At step 1220, a desired process is described in the script editor in terms of a predetermined language. If the description has no error, it is stored (step 1230). When an item "execute" is selected from the main control menu 120, a video image is reproduced, a frame for which a process has been defined is displayed, and at the same time the defined process is executed.