An image processing device includes a mode instruction information obtaining section, a color determination threshold storage section, a mode determination section, and an automatic color determination threshold adjustment section. The mode determination section, using a color determination threshold, determines a mode to be a color mode when the image data belongs to a color mode range, and determines the mode to be a monochrome mode when the image data belongs to a monochrome mode range. The automatic color determination threshold adjustment section adjusts the color determination threshold so as to widen a mode range corresponding to the mode determined by the mode determination section, when the mode indicated by the instruction information obtained by the mode instruction information obtaining section is different from the mode determined by the mode determination section.