PURPOSE: To eliminate in real time the noise of a background region by combining a logic element and a shift register which stores plural picture element information trailing or preceding a noticed picture element.

CONSTITUTION: A shift register SR3 stores N units of picture element information within a mask region MASK2. Now it is supposed that the information on a noticed picture element B1 and its trailing (N-1) units of picture elements B2WBN are stored in the register SR3. In such a case, the information on the element B1 is added to a gate AND4 with other information applied to a gate AND3 respectively. Therefore the information on the element B1 is delivered via the AND4 when the information on the elements B2WBN are all set at 1. Otherwise 0 is delivered as the picture element information on the element B1. Thus the noise of the background region can be eliminated in real time. The same procedure is also effective even in case the information on the picture element preceding the element B1 is stored.