To provide an image processing unit, having flexibility that can be provided by software and high speed processing performance that can be provided by hardware, which quantizes multi-image data into image data of pseudo medium tone.

A pseudo medium tone processing section 122 receives pixel data one by one line each, and stores pixel data for each pixel of a received line, threshold level data decided depending on a position of each pixel, and error data that includes binary error of the pixel data of a just preceding line of the pixel and their adjacent pixel data at a prescribed ratio. The stored data are used, a prescribed ratio of the error data between the pixel data of a just preceding line of the pixel and the adjacent pixel data is included in each pixel of the received line, and the result is compared with the threshold level data, and then the result is binary-processed and outputted as pseudo medium tone data.