An image processing device for synthesizing a plurality of mutually adjacent and partially overlapping pieces of image data, includes a boundary portion selection unit which, from boundary portions of adjacent image data, selects the boundary portion for calculating an image correction value, a correction value calculator which calculates the image correction value of the boundary portion selected by the boundary portion selection unit, and an image correction unit which, in the boundary portion selected by the boundary portion selection unit, performs image correction of the boundary portion using the image correction value calculated by the correction value calculator, and which, in the boundary portion that has not been selected by the boundary portion selection unit, performs image correction using the image correction value calculated in the past.