PURPOSE: To prevent the omission of a thin line of 1 dot width even if a thinning processing is performed by extracting the feature of a noticed block of blocked image data and forming image data whose recording density is low in accordance with the feature.

CONSTITUTION: A CPU investigates each 2×2 bit area (total 4 bits) of work memory areas, defines the bit corresponding to a page memory area as 1 if at least 1 bit in the area is 1 and defines the bit corresponding to the page memory area as 0 if all the bits in the area are 0. For instance, if even only one black dot is contained in a 600dpi 2×2 dot area, the area is converted into 300 dpi black dot, for instance. Therefore, the omission of a thin line of one dot width, for instance, generated by a conventional simple thinning can be prevented. Namely, a thin line (a) of 600dpi 1 dot width is converted into a thin line (b) of 300dpi 1 dot width, for instance.