1. An image processing device comprising: an image acquisition unit for acquiring a normal light observation image captured by an endoscope using white light as observation light and a special light observation image captured by the endoscope simultaneously with the normal light observation image using predetermined narrowband light as the observation light; and an image processing unit for subjecting the normal light observation image acquired by the image acquisition unit to predetermined processing to generate a processed normal light observation image and compositing information of the processed normal light observation image with the special light observation image, wherein the image processing unit has a function of: processing the normal light observation image with a low-pass filter to generate the processed normal light observation image; determining a ratio between red, green and blue in each pixel of the processed normal light observation image and an average of the ratio between red, green and blue; extracting pixels in which a difference from the determined average of the ratio is equal to or larger than a predetermined threshold; and compositing an image composed of the extracted pixels with the special light observation image.