To improve a detection rate of a dot area.

A peak point arithmetic circuit 11 operates the density difference between a marked pixel and its ambient pixels in each marked pixel ' a coexisting image 1.ike a character, a dot, etc., and a primary decision storage circuit 12 makes a primary decision of whether marked pixel density is a peak value or not based on the density difference. In addition, a dot cyclic arithmetic circuit 13 operates how many peak values exist within twelve pixels and even if one peak val.ue exists, it ma)ces,it a dot candidate. A dot continuity arithmetic circuit 14 operates how many peak val.ues exist within thirty-eight pixels to the dot candidate. A secondary decision circuit 15 decides whether peak values exist 2 ninety percent within the thirty-eight pixels. Furthermore, final decision and storage circuits 16 extract a marked pixel area as a dot image of $ one hundred lines and outputs area separation data.