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, wherein the extractor (i) processes the first extraction data and the second extraction data to display a combined image of the left and right images, (ii) incorporates boundary data into the output image data to show a boundary line, which extends horizontally to indicate a boundary between the first and second regions, and (iii) incorporates measurement image data into the output image data to overlap a measurement image on the boundary line, the measurement image data representing a measurement image to be used for measuring a positional deviation amount between the left and right images, and wherein the measurement image includes a horizontal line which is thicker than the boundary line and is overlapped with the boundary line.