1. A multiprojection system that displays one color image by joining a plurality of color images projected by respective projectors, the system comprising: image capturing means comprising optical means having a plurality of filter characteristics corresponding to a plurality of basic colors constituting the color image projected by the projectors, and capturing an image of each of the basic colors via the optical means; and calculating means for calculating offset correction data for each of the basic colors on the basis of a luminance distribution of offset light of each of the basic colors obtained by allowing the image capturing means to capture a black level image projected by the projectors.