A pattern matching processing unit 10 binarizes input image data 1, compares a binarized pattern with a pattern provided for each of groups, and outputs either first information indicating a group to which a pattern that matches the binarized pattern belongs or second information indicating no match. A 0-degree direction dedicated filter 21, a 45-degree direction dedicated filter 22, a 90-degree direction dedicated filter 23, and a 135-degree direction dedicated filter 24 which are disposed correspondingly to the groups carry out smoothing processes according to the direction of an edge of the image, respectively. When the pattern matching processing unit outputs the first information, a selector 30 selects the output of either one of the filters corresponding to the group as output image data 2, whereas when the pattern matching processing unit outputs the second information, the selector outputs the input image data 1 as the output image data 2.