1. An image processing method of obtaining a clear image from a blurred image, comprising: converting an array of image data from a two dimension into a one-dimensional vector, the image data storing pixel values of the blurred image; converting all of two-dimensional matrix operations acting on the one-dimensional vector into an image filtering for the image data of the blurred image, and calculating values of statistic parameters necessary for calculation of a cost function estimating the clear image and a gradient vector of the cost function; and obtaining, by using the gradient vector being calculated, image data of the clear image by a numerical optimization method which calculates the image data of the clear image by using only the gradient vector such that a value of the cost function becomes maximum or minimum.