An image processor, includes: a gradation direction determination unit that determines a gradation direction existing in an image in which a color or density changes; a reference area setting unit that sets a shape of a reference area based on a determination result by the gradation direction determination unit; and a color estimation unit that estimates a color in which a non-target image superimposition does not occur based on a pixel value within the reference area set by the reference area setting unit and substitute an input pixel value with the estimated color.