1. An image processing device comprising: a brightness information calculation portion that calculates brightness information of an image signal that is input and that can form a two-dimensional image made from pixels; a normal direction estimation portion that estimates a normal direction for each pixel, or each region made of a plurality of pixels, from the brightness information in order to give a feeling of depth to the two-dimensional image that is formed by the image signal, and obtains a normal direction vector that is a three-dimensional vector for each pixel or for each region based on the normal direction that is estimated; and a corrected gradation derivation portion that finds a correction amount of the brightness information based on the normal direction vector that is obtained by the normal direction estimation portion, and corrects the brightness information of the image signal based on the correction amount of the brightness information.