An image processor and an image processing method which can minimize variation of color tone or density in an image after screen processing, even if semi-transparency has been specified. The image processor generates semi-transparent image data by overlaying a semi-transparent object on the PDL data to be rendered semi-transparent. Subsequently, screen processing is performed on the semi-transparent image data, by dither processing. Subsequently it is determined whether or not to define the screen processed semi-transparent image data as the image data for printing. If the result of determination is No, the halftone value of the semi-transparent object is modified to be larger than the current value.