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; a setting unit configured to set a parameter for deformation processing by said deformation unit; and a receiving unit configured to receive, while recognizably projecting four corners of an overlap area in a projection image by the projector, user's operations to input instructions to select one of said four corners and to move the position of the selected corner, wherein the overlap area in the projection image is predetermined as an area to be overlapped with a part of an adjacent projection image projected by another projector, wherein said setting unit sets as said parameter for deformation processing a parameter for deforming an entire projection image to be projected by the projector based on coordinates of said four corners after movement of the selected corner, and said deformation unit carries out a deformation based on the set parameter.