1. An image processing device comprising: image input means for inputting at least two images captured at different points of time; setting means for setting an image processing condition for enhancing temporal change based on information associated with the at least two images received from the image input means; image processing means for applying image processing to the at least two images to enhance the contrast of a given range including a target pixel in each of the two images nonlinearly as an average pixel value in the given range decreases on the basis of the image processing condition; and difference processing means for performing difference processing on the processed images to generate a difference image, wherein the image processing means calculates an average image of each of the at least two images by applying an averaging process to the at least two images, the averaging process comprising replacing the target pixel with an average value of adjacent pixels in the given range including the target pixel, wherein the image processing means multiplies a value of a pixel of each of the two averaged images corresponding to coordinates of a pixel of the average image by a coefficient determined depending on a value of the pixel of the average value.