To provide an image data processing apparatus capable of executing complicated image processing at a high speed even when the processing performance on an image input side is low.

A mobile phone 10 acting like a first apparatus receives image data to which visually unrecognizable steganographic data are embedded and transmits the data externally, and receives and stores an external processing result. A server 12 acting like a second apparatus applies data processing to image data received from the mobile phone 10 to acquire the steganographic data and transmits the acquired steganographic data to the mobile phone 10 as a processing result.