1. An image data processing device for generating output image data which represents a stereoscopic 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, comprising: an extractor configured to extract a part of first image data representing a first image as first extraction data, which represents the first region image, and a part of second image data, which represents a second image to be viewed and compared simultaneously with the first image, as second extraction data representing the second region image, wherein the first image is one of a left image to be viewed by a left eye and a right image to be viewed by a right eye, wherein the second image is another of the left and right images, and wherein the extractor processes the first extraction data and the second extraction data and incorporates measurement image data into the output image data, the measurement image data representing a measurement image for measuring a deviation amount between the left and right images.)