To restore an image with no high-degree algorithm required by performing the image restoration while shifting a degradation parameter even when the degradation parameter forming a degradation function needed for the image restoration is unknown and then deriving a degradation parameter having the highest degree of restoration of every image.

An image is generated for a repeated operation (2-1) and then a degraded image is restored while the degradation parameter forming a degradation function is varied (2-2 to 2-4). A degradation parameter having the highest degree of image restoration is selected and the degradation function is derived (2-5). Then a restored image is generated, based on the selected degradation function (2-6).