1. An image processing method for removing a noise component contained in an original image, comprising: smoothing a noise component contained in an original image on a temporary basis by using a noise fluctuation index value; extracting a temporary noise component free of a non-noise component based upon a differential signal representing a difference between the original image and a smoothed image; extracting an actual noise component by further excluding a component less likely to be a noise component from the extracted temporary noise component by comparing the extracted temporary noise component with the noise fluctuation index value again; and removing noise from the original image based upon the extracted actual noise component having been extracted.