There is provided an image processing device capable of recognizing a presence of an object with low visibility on a combined image, a warning device and a method for processing an image. The image processing device (13) includes an image acquiring unit (18) configured to acquire a first image capturing an area around a mobile object (17) including a first area and a second image capturing an area around the mobile object (17) including a second area adjacent to the first area; a contour detector (19) configured to perform a contour detection to the first and second images; and a determiner (20) configured to detect, if a first contour detected on the first image extends to a boundary with the second image on a combined image generated by using the first and second images, whether or not a second contour is detected on an extension of the first contour on the second image.