This invention re-encodes encoded data (e.g., PackBits encoded data), which is expressed by a data format of a runlength code part indicating the runlength of the same data and a data part indicating the data, and a runlength code part indicating the runlength of a different data stream and a data part indicating the different data stream, to the same data format without decoding that encoded data, thereby improving a compression ratio. To this end, upon reception of data encoded by a PackBits encoding process, a data division unit separates that data into information indicating the runlength of data, and a data part, and outputs them as num and data. A data processing unit masks a predetermined bit in accordance with an instruction from a monitor unit, and outputs that result to a data combining unit. The data combining unit and a data output unit reconstruct and output data in the PackBits format in accordance with the masked data part and num data.