1. An image processing method for calculating from an absolute-value image produced by complex observed signals S an average value M of the original complex observed signals S in a local region on the absolute-value image, comprising the step of: calculating the average value M by: M&equals;&agr;·ƒ −1 ( Ma/&agr; ), wherein the function Ma/&agr;&equals;ƒ(M/&agr;) is represented by: Ma/&agr;&equals;∫∫&lcub;&verbar;M/&agr;&plus; ( X&plus;iY )&verbar;exp&lcub;−( X 2 &plus;Y 2 )/2&rcub;/2&pgr;&rcub; dX dY, the standard deviation of noise contained in the complex observed signals S is represented by &agr;, and the average value of pixel values in the local region is represented as Ma.