PURPOSE: To surely extract a picture having a certain density or a certain density range by removing noise from it by scanning a gradation picture by an operator consisting of a center picture element and peripheral picture elements and extracting a picture element in the gradation picture satisfying a specific condition.

CONSTITUTION: In the case of scanning a gradation picture by the operator 5 and extracting picture elements in the range of density values α to β by the center picture element 6, density gradient among adjacent picture elements is found out by peripheral picture elements 7, 8. Namely the gradation picture is scanned by the operator 5 from the uppermost picture element in the descending order in each picture element. When it is defined that the density of the center picture element 6 out of many picture elements in the gradation picture is A, a picture element satisfying two conditions, i.e., α<A<β and A-B<K provided that the density of the 1st peripheral picture elements 7 is B and a density gradient value between A and B is A-B, is extracted. Although the K is properly determined by the density range of an area to be extracted, a value less than (β-α) is preferable.