To make it impossible to visually notice undesired moire stripes generated at the time of displaying halftone-processed binary data.

Image data 10 are expressed with plural scanning lines, and each pixel of each scanning line indicates optical concentration at the specific place of the image data 10. The pixel pairs in a high speed scanning direction of the odd-numbered scanning lines are grouped into odd/even pixel pairs, and the even-numbered pixel pairs are grouped into odd/even pixel pairs, and the groups in the adjacent scanning lines are shifted by a logic circuit 12. Each pixel pair includes (n) addressable units deciding the outline of a halftone cell. A storage place in a lookup table 14 including a corresponding binary data value indicating the intensity level of each pixel is addressed by using the combination of an error value propagated from the pixel pair with the data value of each pixel pair. The binary data are outputted to a gray modulator 16 for printing the binary data by a printer 18. Outputted data grating is equivalent to a halftone screen with extremely high frequencies in which moire stripes can not visually noticed.