To satisfactorily convert the resolution of an input image while suppressing degradation in picture quality after resolution conversion by synthesizing correspondent patterns coincident with two predetermined thresholds and resolution converted image data when an area suitable for the patterns set by using the respective thresholds is detected.

The inputted image data of resolution 400dpi, for example, are stored from an input terminal A to a line buffer 11 successively from the leading picture element of a leading line and outputted to a suitable pattern detector 12 and a pattern separator 13. When any one of 18 kinds of patterns is detected, for example, at the pattern detector 12, the value of a concerned picture element in that pattern is inverted by the pattern separator 13 and inputted to a resolution converter 14. Binary image data of which the resolution is converted to 600dpi, for example, from the resolution converter 14 and the binary pattern data of 600dpi selected by a pattern generator 15 are synthesized by a synthesizer 16 and outputted to an output terminal B.