1. An image evaluation apparatus comprising: a block setting unit that sets a block, which has a predetermined size, in an input image; a difference evaluation unit that evaluates a gradation difference of the input image based on the block set by the block setting unit; a control unit that controls the block setting unit and the difference evaluation unit so as to evaluate the gradation difference of the input image with respect to plural types of blocks, which are different in phases in the image or in size; and an evaluation value generating unit, wherein the difference evaluation unit includes a flat-pixel-region judgment unit that judges as to whether or not an image region contains an edge, based on a gradation change amount of the input image, a pixel extraction unit that extracts, from the image region of the input image which is judged by the flat-pixel-region judgment unit not to contain the edge, a pixel regions including (i) a pair of block-boundary pixels in a boundary position of coding blocks and (ii) a pair of non-block-boundary pixels in a position other than the boundary position of the coding blocks, an intra-pair difference calculation unit that calculates a difference between the pair of block-boundary pixels of the extracted pixel region as a first difference of the extracted pixel region, the intra-pair difference calculation unit that calculates a difference between the pair of non-block-boundary pixels of the extracted pixel region as a second difference of the extracted pixel region, an inter-pair difference calculation unit that calculates a difference between the first and second differences of the extracted pixel region as an amount of block distortion of the extracted pixel region, and an evaluation unit that evaluates an amount of block distortion of the input image on a basis of the calculated amount of block distortion of the extracted pixel region, the evaluation value generating unit generates an evaluation value, regarding the gradation difference, of the input image based on (i) the gradation differences, which are evaluated by the difference evaluation unit with respect to the respective plural types of blocks, and (ii) the amounts of block distortion, which are evaluated with respect to the respective plural types of blocks.