1. An image processing apparatus comprising: an image compression conversion circuit which performs compression processing and low resolution conversion on an input image to generate a compressed image; an image memory which stores the compressed image; an image expansion conversion circuit which performs expansion processing on the compressed image read out from the image memory and performs resolution conversion to return a resolution thereof to an original resolution; and an edge classification circuit which detects an edge pattern included in an expanded image when the compressed image is expanded and classifies an edge according to the edge pattern; wherein: the image compression conversion circuit: (i) judges whether each block unit, which comprises a plurality of pixels in the input image, is a half tone region or a high resolution region according to a predetermined condition, (ii) converts data values of the plurality of pixels included in each block unit into a data value of one pixel by quantizing the data values according to a predetermined condition depending on whether the block unit is judged as the half tone region or the high resolution region to generate quantized data which comprises one pixel in the compressed image, and (iii) sets identification data in each pixel in the compressed image to identify whether the pixel is the half tone region or the high resolution region; and the edge classification circuit, with respect to each pixel in the compressed image, (i) judges whether the identification data and the quantized data of a watching pixel and a surrounding pixel in a predetermined region from the watching pixel fit a predetermined condition of edge pattern, (ii) detects a pixel in the expanded image which corresponds to the watching pixel as the edge pattern when the identification data and the quantized data are judged as fitting the predetermined condition, and (iii) performs edge classification of the pixel in the expanded image which corresponds to the watching pixel according to the fitted predetermined condition.