To eliminate moire pattern and rough feeling from an output image at the time of semitransparent processing of an object.

The image processor 1 for interpreting a page description language delivered from a host computer H and converting it into print data in print output format comprises a section 12 for making a decision whether the page description language performs semitransparent processing of a foreground image and a background image by logical operation or not, and a semitransparent processing section 16 for calculating the average value of pixels corresponding to the foreground image and background image when a decision is made that semitransparent processing is performed by logical operation to obtain semitransparent image data.