1. An image processing device that sequentially generates a synthetic image by joining a first image, which is a single image or is constituted by a plurality of the images joined together, and a second image that is input, every time when the second image is input, the device comprising: an overlapping region obtainment portion that obtains a motion vector on the basis of the second image and an image input immediately before the second image among the images constituting the first image, and obtains an overlapping region where the image input immediately before and the second image overlap each other on the basis of the obtained motion vector; a brightness-value conversion factor calculation portion that calculates a brightness conversion factor for converting a brightness value of the first image or a brightness value of the second image on the basis of the brightness value of the first image and the brightness value of the second image in the overlapping region so that a change in the brightness values between the first image and the second image becomes small; and a synthetic image generation portion that generates the synthetic image by converting the first image or the second image using the brightness conversion factor and by joining the first image and the second image.