A portable terminal detects a color change region corresponding to a marker in frames, and determines whether the brightness and/or hue change with time in the color change region matches a search rule. Subsequently, when the brightness and/or hue change with time in the color change region matches a search rule, the portable terminal restores information by decoding and determines whether the information is the content of predetermined information corresponding to the search rule determined to match. Furthermore, when the restored information is the content of predetermined information corresponding to the search rule determined to match, the portable terminal displays the information in association with the color change region to provide notice that the corresponding flying object is an appropriate flying object performing optical communication.