An image formation apparatus includes an aperiodic data generating unit for generating aperiodic data between pixel areas having a neighboring relation every pixel area comprising a plurality of pixels having an interlocking relation of the image data, a threshold value deciding unit for converting the aperiodic data into a plurality of threshold data for forming a systematic dot array in the pixel area, and a gradation conversion processing unit for performing a gradation converting process by comparing the image data with the threshold data, thereby reducing a signal processing amount as compared with that in case of performing the propagating process on a pixel unit basis.