1. An image processing device comprising: an image processing unit for synthesizing a plurality of original images to generate a reference image, calculating a similarity for each of the plurality of original images by comparing with the generated reference image, smoothing the plurality of original images based on the calculated similarity, and synthesizing the plurality of smoothed original images to acquire a synthesized image wherein the image processing unit comprises: a band dividing section for dividing the plurality of original images for each predetermined wavelength band to generate a plurality of wavelength-band original images; a reference image generating section for synthesizing the plurality of wavelength-band original images for each of the predetermined wavelength bands to generate the reference image for each of the wavelength bands; a similarity calculating section for comparing each of the plurality of wavelength-band original images with the reference image in the same wavelength band to calculate the similarity for each of the wavelength-band original images; a denoising section for generating denoised images from each of the wavelength-band original images by smoothing each of the plurality of wavelength-band original images using the similarity of the said wavelength-band original image; and a synthesizing section for synthesizing the denoised images to generate the synthesized image.