1. An image processing method for restoring input N-level image data to M-level image data by estimating an original image, said method comprising the steps of: determining coupling coefficients between neurons of a neural network by repeatedly performing the following steps (a)-(e): (a) selecting a first object pixel of N-level image data, M-level image data corresponding to which is known; (b) inputting the selected first object pixel and surrounding pixels of the first object pixel of the N-level image data to input neurons of the neural network as input image data, each of the input neurons having been previously determined to input specific data of the first object pixel and the surrounding pixels; (c) inputting a second object pixel of the M-level image data corresponding to the first object pixel of the N-level image data as an ideal output for said input image data to the neural network; (d) comparing an output of the neural network with the ideal output; and (e) correcting the coupling coefficients on the basis of a result of the comparison; successively selecting each of the pixels of an N-level image to be restored as a third object pixel; inputting the selected third object pixel and surrounding pixels of the third object pixel of the N-level image to the respective input neurons of the neural network; and outputting output image data of the neural network corresponding to the third object pixel as an estimated M-level image.