When image data is converted into an expression format based on the dot on-off state, such conversion is performed in a unit of a raster group comprising a predetermined number of adjacent rasters. When this is done, the binarization errors occurring in each pixel of the last raster located in the last position of such raster group are diffused into surrounding pixels and stored in a first storage unit. The errors are read out from the first storage unit for dot on-off state determination regarding the first raster of the raster group adjacent to the above-mentioned last raster, and binarization errors occurring in connection with this determination are stored in a second storage unit that permits faster reading and writing of data than the first storage unit. The remaining rasters following the first raster are converted into dot rows by conducting dot on-off state determination for each pixel therein in parallel with the process to convert the first raster into a dot row while taking into account the binarization errors that occurred in the first raster. Consequently, the errors that are taken into account by the pixels of the same raster are stored in the second storage unit that permits faster high-speed reading and writing of data, and the image data for the raster group can be converted at a high speed.