To provide a processing method for binary image data by which an image forming device such as an LED printer, where image forming elements are arranged fixedly in a main scanning direction, can clearly form an image at high-speed.

In the processing method for binary image data applied to the image forming device having a recording head 50 where a plurality of pixel forming elements are arranged in the main scanning direction and driving each pixel forming element of the recording head 50 on the basis of binary image data to form a desired image, the received binary original image data are recognized by a pattern of a grating matrix of n×n or n×m (n, m are number of bits), the pattern is compared with a plurality of discrimination patterns stored in advance, interpolation image data with pixels of a smaller dots than those of the pixels of the original image are generated according to a plurality of discrimination patterns stored in advance, and the original image data and the interpolation image data are alternately outputted by the recording head at a period of 1/2 line period so as to conduct smoothing processing to be applied.