1. An image processing apparatus comprising: edge judging means for judging a presence or absence of an edge pixel for input image data, wherein the edge judging means includes a left edge pixel detecting means having an output connected to an input of a left edge expanding means, and a right edge pixel detecting means in parallel with the left edge pixel detecting means, and having an output connected to an input of a right edge expanding means, the left and right edge pixel detecting means receiving the input image data in parallel, wherein said left edge expanding means expands a left edge pixel detecting result toward a pixel at a front side in a main scanning direction and said right edge expanding means expands a right edge pixel detecting result toward a rear side in the main scanning direction, and wherein the edge pixel detecting results are respectively input into an OR circuit and the presence or absence of the edge pixel is based on taking a logical sum of the expanded left edge pixel detecting result, which is delayed based on a difference between expanding directions in the left edge and right edge pixel expanding means, and the expanded right edge pixel detecting result; and plural halftone processing means, wherein one of said plural halftone processing means is selected in accordance with the judgment result of said edge judging means.