To further increase a coding rate by coding input data at the longest possible period within the period at which the input data matches best with dictionary data.

A data processing apparatus is configured to code image data line by line. An image reading section 22 reads in image data line by line from a main memory 10. As to the image data read in, an MTF data width selection section 23 decides the longest period and an MTF data width segmentation section 28 segments it accordingly. The segmented data is next converted to intermediate data in an MTF processing section 29, and the intermediate data is Huffman-coded in a Huffman coding processing section 30.