1. A projector which constitutes an image projection system to project one image by combining a plurality of projection images projected by a plurality of projectors on a screen while overlapping parts of the projection images with each other, said projector comprising: a deformation unit configured to carry out geometric deformation with respect to an image to be projected therefrom; and a setting unit configured to set a parameter for deformation processing by said deformation unit; and wherein said setting unit sets said parameter for deformation processing based on a relation between a position of an overlap area in the projection image before the deformation by said deformation unit is carried out, and a position of an overlap area in the projection image after the deformation by said deformation unit has been carried out.