PURPOSE: To perform satisfactory smoothing processing to images mixing the images of characters or drawings and pseudo half tone images by holding the gradation of the pseudo half tone images while inhibiting the smoothing processing in the case of a pattern whose appearance probability in the pseudo half tone images is high.

CONSTITUTION: A smoothing processing part 6 is internally provided with a CPU, ROM and RAM or the like and performs picture element judgement while catching pixel data a1, b1, c1, a2, b2, c2, a3, b3 and c3 fetched from respective DFF circuits 1, 3 and 5 as patterns. In this case, the data b2 are defined as attention pixel data and the data a1, b1...c3 are defined as reference pixel data. After the desired smoothing processing is performed, this processing part 6 converts one bit of source image data A to four bits and outputs them as converted image data B. The processing part 6 is connected the line of the input picture element of 9 bits to the address of the ROM and in the case of the pattern whose appearance probability in the pseudo half tone image is high, the ROM holds the gradation of the pseudo half tone image by inhibiting the smoothing processing.