1. An image processing method of image processing of a face image, comprising steps of: processing said face image monochromatically by gray scale processing; segmenting a first image area inclusive of a skin area of a face from said face image after said gray scale processing; specifying a second image area inclusive of said first image area and also a hair area of said face in said face image; generating a brightness histogram of said first image area; extracting a shadow point and an eyeball point of said face from said first image area, said shadow point having a lowest brightness in said skin area; generating a modified histogram by modifying said brightness histogram so as to correct a specific area having a lower brightness level than said eyeball point at a brightness level of said eyeball point, and to correct a specific portion in said skin area having a higher brightness level than said shadow point at a brightness level of said shadow point; determining one to three thresholds from said modified histogram; thresholding said second image area by use of said one to three thresholds to produce an N-level encoded image, where N is an integer of 2 or more.