Plural pieces of reference data are stored for plural pixel arrangement patterns. Each piece of reference data indicates which of first-type edge pixels needs to be subjected to determination to determine whether an outside neighboring first-type pixel adjacent to the first-type edge pixel exists. A piece of reference data corresponding to a target pixel block is specified as target reference data. A reference-data-based judgment is executed on the target pixel block to perform the determination onto those first-type edge pixels that are indicated by the target reference data as need to be subjected to the determination. A correction is executed on the target pixel block to correct the type of at least one first-type pixel contained in the target pixel block into the second type in a case where a result of the reference-data-based judgment indicates that no outside neighboring first-type pixel exists with respect to the target pixel block.