1. A multimedia contents converting apparatus configured to convert input multimedia contents to be displayed on a screen with a predetermined size, including text media and media other than the text media, into output multimedia contents to be displayed on a screen with a size smaller than said predetermined size, said apparatus comprising: a text display time calculation processing unit for calculating a display time period of text media to be included in said output multimedia contents, on the basis of conversion instruction information; a spatiotemporal layout information setting processing unit for setting spatiotemporal layout information on said output multimedia contents on the basis of said conversion instruction information; and a contents conversion processing unit for converting said input multimedia contents into said output multimedia contents on the basis of said spatiotemporal layout information, said spatiotemporal layout information setting processing unit setting a display time period of said text media included in said spatiotemporal layout information, on the basis of said display time period of said text media calculated in said text display time calculation processing unit, and setting a playing time period of said media other than said text media included in said spatiotemporal layout information, on the basis of said display time period of said text media set as said spatiotemporal layout information.