An image data processing apparatus, increase: a reader that reads image data included in an image file in read units of a specified data length from an external storage medium storing the image file; a storage that temporarily saves the image data read by the reader; and a data processor that requests image data to the reader and applying a specified data processing to the image data given from the reader in response to the request; wherein the reader reads image data requested by the data processor from the storage and supplies them to the data processor and, if the image data requested by the data processor cannot be read from the storage, reads image data of a specified data length including the image data from the storage medium and supplies them to the data processor, and the reader optimizes the read unit according to the form of the request from the data processor.