1. An image processing device for processing target image data, comprising: a computer-readable medium having stored thereon target image data generated through decoding of respective pixel blocks including a plurality of pixels; a processor configured as a color component acquisition unit that reads target image data from the computer-readable medium and acquires a luminance component, a first color difference component, and a second color difference component which represent color of respective pixels; a processor configured as a smoothing processor that, at individual pixel positions of at least a partial area of a target image represented by the target image data, carries out a maximum magnitude smoothing process on the first color difference component, among the luminance component, the first color difference component, and the second color difference component, and writes processed target image data to the computer-readable medium; a processor configured as a noise strength calculator that, in relation to a specific color component among the color components targeted for a smoothing process including the first color difference component, calculates block noise strength at a boundary line between a target pixel block within the target image data and another pixel block neighboring the target pixel block by a predetermined noise strength calculation using the target image data; wherein the smoothing processor, in relation to the specific color component, carries out a stronger smoothing process in a case where the block noise strength is greater than a strength threshold value as compared with a case where the block noise strength is less than the strength threshold value for at least a partial area within the target pixel block; and wherein the noise strength calculation is set so as to calculate weaker block noise strength in a case where an index is greater than an index threshold value as compared with a case where the index is smaller than the index threshold value, the index indicating a magnitude of a variation rate of pixel values within the target pixel block.