An image processing device for estimating the three-dimensional size of a foreign object is provided. The image processing device includes a storage section configured to store respective first pixel values of a plurality of pixels; a pixel value computing section configured to calculate the respective second pixel values of the plurality of pixels, the second pixel values being each a ratio of the difference between the first pixel value of the corresponding pixel and the background value of the corresponding pixel to the background value; and a pixel value integrating section configured to integrate respective second pixel values of a group of pixels belonging in a continuous region.