1. An image processing apparatus for converting a color image into a monochrome image, the image processing apparatus having a reading unit for reading an original, wherein the image processing apparatus comprises: determining means for determining whether a color image is formed by reading an original with the reading unit or input from an external apparatus; setting means for setting a conversion method for converting the color image into a monochrome image in such a manner that when an image obtained by monochrome converting the color image is output onto a recording medium, security information included in the color image is difficult to be discriminated by human eyes, if the determining means determines that the color image is formed by reading an original with the reading unit, wherein the setting means sets a monochrome conversion method selected by a user, if the determining means determines that the color image is input from an external apparatus; and converting means for converting the color image into a monochrome image by the monochrome conversion method set by the setting means.