PURPOSE: To improve gradation and resolution by providing an average density detecting means to detect an average density value from a binary data group in a range corresponding to the density level of attention picture element data and weighting allocated to respective picture element positions in this range, and a binarizing means to binarize the attention picture element data with the density, which is detected by the average density detecting means, as a threshold value.

CONSTITUTION: Average density calculation ROM 8 and 9 are ROM to calculate the average density in the prescribed area from the binary data around an attention picture element. Then, the ROM 8 outputs an m1 (i, j) and the ROM 9 outputs an m2 (i, j). A comparator 10 compares multilevel data to the attention picture element with a prescribed value β. A selector 11 selects any one of plural weighted average values as the threshold value based on a compared result signal (select signal) from the comparator 10. A comparator 11 outputs binarized data B (i, j). The binarized data are inputted to delay RAM 2 and 3 for delaying the data for each line and the binary data delayed for one line are outputted by the delay RAM 2. Then, the binary data delayed for two lines are outputted by the delay RAM 1.