1. An image projection system which projects a plurality of same images on a plane of projection by superimposition, comprising: a plurality of projectors configured to have mutually different resolutions and project the plurality of same images on the plane of projection; and a processor to output to the plurality of projectors image data corresponding to the plurality of same images with the resolutions of the plurality of projectors, respectively, wherein the resolutions of the image data are preset to a highest resolution of a first projector among the resolutions of the plurality of projectors, and the processor includes a resolution converter configured to convert the highest resolution of image data to be output to one of the plurality of projectors other than the first projector into a resolution of said one of the plurality of projectors.