PURPOSE: To obtain a distortionless reduced picture by extending object data of logical operation into four lines or columns.

CONSTITUTION: In case that the l-th line (or column) of dots is eliminated, data of the (l-1)-th line, the l-th line, the (l+1)-th line and the (l+2)-th line, namely, four lines (or four columns) is utilized. Then, AND between l and l+1 is operated when l-1 and l+2 are "1"; and OR between l and l+1 is operated when one of l-1 and l+2 is "0" at least; and obtained data is substituted as data in l and l+1 positions.