1. An image processing device comprising: an estimating module configured to estimate boundaries of blocks in an input image, the blocks being utilized as units of in-frame transform when the input image was coded; a characteristic value calculating module configured to calculate a characteristic value of a subject pixel group having constituent pixels including a pixel in the frame and at least one pixel that is adjacent to the pixel by calculating: (a) a degree of pixel value dispersion between the constituent pixels of the subject pixel group if all the constituent pixels belong to the same block; and (b) degrees of pixel value dispersions each obtained between each of the constituent pixels of the subject pixel group and a pixel that is adjacent to the constituent pixels and belongs to the same block as the constituent pixels if the constituent pixels belong to two or more distinct blocks; a strength value calculating module configured to calculate, from the characteristic value, a strength value to control the strength of smoothing between the constituent pixels of the subject pixel group; and a smoothing module configured to perform pixel value smoothing between the constituent pixels of the subject pixel group according to the strength value calculated by the strength value calculating module.