1. An image data enlarging/smoothing processor for carrying out an interpolation process of pixels simultaneously with a pixel density conversion of binary image data, comprising: connective pattern detecting means for detecting a form of a connective pattern of reference pixels adjoining to a marked pixel; enlarging/smoothing processing means operating such that said processing means enlarges the marked pixel on the basis of enlargement ratios in fast and slow scan directions, thereby enlarging and smoothing the marked pixel on the basis of the form of the connective pattern of the reference pixels and processing positions in the fast and slow scan directions; said connective pattern detecting means detecting forms of four directions of a black-pixel connective pattern of 1:1 and forms of eight directions of black-pixel connective patterns of 1:2 to 1:n (n being a positive integer); and said enlarging/smoothing processing means enlarging the marked pixel in accordance with an enlargement ratio, and interpolating the enlarged pixel in accordance with the detected form of the connective pattern.