In this paper, we present a method for geometric calibration of a multi-projector display system. It enables easy calibration of the system such that the user needs only to take a picture of the projected images on the planar screen with a hand-held camera to accomplish entire calibration. Using the calibration method, one can realize a large high-resolution image display by placing multiple projectors on a desk etc. in an arbitrary manner. The calibration requires four or more projectors and includes not only alignment of the images but overall rectification of the stitched image. The problem to be solved is to recover the Euclidean structure of the system (at least partially); its difficulties arise from the fact that the intrinsics of the projectors, especially the focal lengths, need to be estimated along with other parameters. For the problem, we show uniqueness of solutions and several critical configurations, which were unclear in previous studies, and then present an algorithm. We present several experimental results that demonstrate the feasibility of the proposed method.