A printer fetches a horizontal direction DCT coefficient group and a vertical direction DCT coefficient group for each block from the JPEG data. The printer selects edge patterns similar to the brightness changes expressed by these coefficient groups from a specified table, and records those pattern numbers in a RAM. The printer judges whether or not the brightness changes of the blocks with each other adjacent are continuous based on the pattern number of each block recorded in the RAM. And by connecting the edge patterns when those brightness changes are continuous, the printer accumulates the blur widths that exist extending over the blocks. Then, based on this cumulative value, the printer determines the presence or absence of image blur. It is possible thereby to detect with good precision the blur of images even for high resolution images while reducing the used memory capacity.