The application discloses an image data processing device for generating output image data which represents an output image including a first region image to be displayed in a first region and a second region image to be displayed in a second region adjacent to the first region. The image data processing device includes an extractor configured to extract a part of first image data representing a first image as first extraction data representing the first region image and a part of second image data representing a second image to be viewed and compared simultaneously with the first image as second extraction data representing the second region image. The extractor processes the first extraction data and the second extraction data to generate the output image data.