To restrict the number of false positives at the time of detecting an abnormal shadow candidate to improve detecting accuracy and shorten a processing time when detecting an abnormal candidate. An image processing device produces image data (D1) by reducing the image data (D) of an original image, produces image data (D2) having its fine low-density area smoothed by subjecting the image data (D1) to a first smoothing filter, produces image data (D3) having almost the same area as that of an abnormal shadow candidate to be detected and having its low-density area smoothed by subjecting the image data (D2) to a second smoothing filter, produces differential image data (D4) by differentiating the image values at the same pixel position of the image data (D3) and the image data (D2), and extracts low-density-area image data (D5) having the size of an object of detection by subjecting this image data (D4) to threshold processing. Then, an area extracted by image data (D5) is used to detect an abnormal shadow candidate.