We are introducing large-scale projection mapping system to school gymnasium aiming to support school activities. Large-scale projection is usually realised by aligning image from multiple projectors. However, the accuracy of alignment will be affected by the projector's lens distortion whose effect is enlarged when a projector is far from the target surface. Traditionally, the misalignment of projector images is corrected manually. In this paper, we purpose a method to perform projector image undistortion as well as alignment automatically by utilizing projector-camera systems.