1. An image processing device comprising: a non-target region detecting unit that detects a region that is not to be examined as a non-target region from an image; a pixel-of-interest, region setting unit that sets a pixel-of-interest region in a predetermined area including a pixel-of-interest position in the image; a surrounding region determining unit that determines a surrounding region, which is an area for acquiring information for use in forming a reference plane with respect to the pixel-of-interest position, based on the non-target region; a reference plane forming unit that forms the reference plane based on the information in the surrounding region; and an outlier pixel detecting unit that detects an outlier pixel having a pixel value numerically distant from circumferential values based on a difference between corresponding quantities of the reference plane at each pixel position and of the original image.