An image processing device includes: an estimated plane calculation portion calculating, for inputted image data, estimated planes defined by luminance values in blocks each containing a predetermined number of pixels including a reference pixel; an optimal estimated plane selection portion selecting an optimal estimated plane having a least summation of errors between luminance values of respective pixels in a block and an estimated plane among the estimated planes; a weighting factor calculation portion calculating a weighting factor for the reference pixel base on luminance values of the reference pixel and in the optimal estimated plane; and a weighted smoothing portion computing a sum of products of relative luminance differences between the luminance values of the respective pixels in the block and in the optimal estimated plane and the weighting factor for the reference pixel and adding a luminance value in the optimal estimated plane at a reference pixel position to a result of computation.