PURPOSE: To eliminate fine grains caused by binarization, etc., by a hardware and to secure the high speed property of the subsequent processing by checking the connectivity extending from the center part to the outside peripheral part and changing a picture element in a prescribed range to a state that there is connectivity, when there is no connectivity.

CONSTITUTION: Four masks I - IV having overlap parts which extend from the respective four corners of an inspection range constituted of (m + 2) ×(n + 2) picture elements and superposed mutually in the center part of its inspection range are set, and one arbitrary picture element in the overlap part is set to a reference picture element. In this state, when any mask has no connectivity extending from the reference picture element to the outside periph ery, the picture element in the range of (m) × (n) of the center part is converted to a state that there is connectivity. When it is decided that each mask has no data connectivity to the picture element of the periphery, it is decided that the range of (m) × (n) centering around the reference picture element is fine grains generated by binarization. Therefore, by converting its part to a state that there is connectivity, fine grains whose size is less than (m) × (n) are eliminated. In such a way, high speed processing can be executed.