1. A method of processing noise in image data by an image processor having a signal-processing portion converting an image signal from an image sensor into a digital signal and outputting the converted signal as image data for each frame, the image data indicating sets of pixel values each having a brightness at a corresponding one of coordinate points arranged in directions of rows and columns, the method comprising the steps of: extracting pixel values by accepting each frame of the image data delivered from the signal-processing portion in turn while shifting the image data at regular intervals of time in a direction of a time axis and extracting, in each frame, pixel values represented by the image data accepted for each frame; deciding pixel value by comparing each pixel value extracted by the pixel values extracting step against a previously set first threshold value and making a decision as to whether the pixel value is less than the first threshold value; finding autocorrelation coefficients of pixel values which are less than the first threshold value and which are possessed by pixels located at the same coordinate point defined by the sets of image data over plural ones of the frames accepted while being shifted at the regular intervals of time; and deciding random noise in the image by making a decision as to whether the autocorrelation coefficients found in the autocorrelation coefficients finding step are less than a previously set second threshold value and determining that the image data has suffered from deterioration of image quality due to random noise when the autocorrelation coefficients are less than the second threshold value.