An object-region-data describing method for describing object region data relating to a time-series variation of an object region in video data including a plurality of frames, the method comprising obtaining a conversion parameter representing conversion from a reference object region into a target object, approximating a time-series variation of the conversion parameter by an approximate function, and describing the object region data using an approximate function parameter identifying the approximate function and information on the reference object region. Thus, it is possible to describe a region of the desired object in video data by a small quantity of data and facilitate generating the object regions and handling data on the object region.