1. An image lightness level estimating method for estimating a lightness level of an image, comprising: dividing an original image into a plurality of image sub-areas according to tone level information of pixels forming the image; computing, by a processor, an average lightness of the plurality of sub-areas producing lightness amounts; computing a statistic amount for estimation of the lightness level of a whole of the original image using the average lightness for each of the plurality of sub-areas; and correcting the original image using the statistic amount.