1. A compression method of quantizing and compressing image pixels, said method comprising the steps of: determining a method for quantizing n×n pixels for each block unit of n×n pixels; quantizing the n×n pixels according to the determined quantization method; storing data of the quantized n×n pixels in a Block Truncation Coding plane region in a memory independent of the quantization method used for the n×n pixels; and storing discrimination data, corresponding to the quantization method used for quantizing the data of the n×n pixels, in the memory, wherein the quantization method used for the n×n pixels differs according to whether individual pixels of the n×n pixels to be quantized are pixels of a half tone area or pixels of a high resolution area when the quantization method is determined, such that (i) pixels of the half tone area are quantized by a Block Truncation Coding compression method, and (ii) pixels of the high resolution area are quantized based on a density pattern formed by the n×n pixels.