For (CMYK) (x,y) data, a target-pixel checking unit calculates a target-pixel integrated value a(x,y) from CMYK binary data of a target pixel and a surrounding-pixel checking unit calculates a maximum surrounding pixel integrated value n(x,y) from respective CMYK binary data of surrounding pixels that are adjacent to the target pixel. Based on the target-pixel integrated value and the maximum surrounding pixel integrated value, a comparison determining unit estimates a higher toner pile height. The comparison determining unit adds transparent image data W(x,y) to the input data CMYK(x,y) and outputs output image data CMYKW(x,y) such that the toner pile heights become even.