Disclosed are an apparatus for providing additional information by using an extension subtitle file including an inverse multiplexing unit to separate video and audio data from a multimedia file input by a data input unit, an XML parser to parse a caption data file and an additional information data file input by the data input unit, video and audio data processing units to decode the video and audio data input by the inverse multiplexing unit, and convert the decoded video and audio data into predetermined video and audio signals, a caption processing unit to process the caption data parsed and input by the XML parser, an additional information data processing unit to process the additional information data file parsed and input by the XML parser, and to generate user interface information based on the processed additional information, and a data output unit to display the processed video, audio, caption and user interface information.