1. An image processing device, comprising: image synthesis processing circuitry that sequentially receives continuous-photographed images photographed under different exposure conditions as an input image, performs a synthesis process of the input image and an intermediate synthetic image serving as a previously processed image, and generates an output image, wherein the image synthesis processing circuitry includes region determining circuitry that classifies configuration pixels of the input image and the intermediate synthetic image into a noise pixel, an effective pixel, or a saturation pixel according to a threshold value, update method deciding circuitry that decides a synthesis processing method of pixel circuitry according to a combination of pixel classification results of the corresponding pixels of the input image and the intermediate synthetic image, and image update processing circuitry that performs a synthesis process of the input image and the intermediate synthetic image according to an update method decided by the update method deciding circuitry, and generates the output image; wherein the region determining circuitry performs a smoothing process on each of the input image and the intermediate synthetic image, calculates a degree of pixel saturation and a degree of pixel noise corresponding to each of configuration pixels of a smoothed input image and a smoothed intermediate synthetic image, and outputs the degree of pixel saturation and the degree of pixel noise to the update method deciding circuitry; wherein the update method deciding circuitry calculates an addition ratio of the following pixel values (a) and (b) using the degree of pixel saturation and the degree of pixel noise: (a) a 3DNR processed pixel value by the pixel value synthesis of the input image and the intermediate synthetic image; and (b) the pixel value of the intermediate synthetic image, and outputs the addition ratio to the image update processing circuitry, and the image update processing circuitry performs an addition process of the pixel values (a) and (b) according to the addition ratio, and decides the pixel value of the output image.