1. An image-processing system for restoring an image F by means of a deterioration function H determined from a captured, deteriorated image G and image-capture conditions, the system comprising: an arithmetic section which computes, as an evaluation value J, a difference between the deteriorated image G and an image HF acquired by subjecting the image F to the deterioration function H and which repeatedly performs processing for computing the image F and the evaluation value J until a gradient of the evaluation value J is the threshold value or less; and a parameter-setting section which sets a convergence parameter multiplied by the gradient to thus adjust the number of iteration operations and sets the convergence parameter to a value which shows an increase and subsequently a decrease with an increase in the number of iterations.