An image data processing apparatus including: a reader that reads image data included an image file from an external storage medium, the image data being variable-length coded data of image blocks The reader performs a first reading processing for reading the image data in an arrangement order thereof from a storage medium and the data processor performs a boundary information derivation processing based on the image data for obtaining boundary information representing a boundary in a data arrangement in the storage medium between the image data belonging to two image blocks located adjacent in the data arrangement in the storage medium and at mutually different positions in the sub scanning direction. The reader performs a second reading processing for reading the image data from the storage medium in an order determined based on the boundary information and saving them in the storage.