1. A projection system comprising: a plurality of projection units configured to project images to form an image as a whole on a projected area; an output unit configured to output images for calibration to projection units that are in charge of areas of corners of the projected area among the plurality of projection units, the images for calibration including positioning markers that are indications for positioning to the corners of the projected area; a detection unit configured to detect, based on one or more captured images for calibration that are acquired by capturing the projected area and the images for calibration including the positioning markers being projected, reference points that characterize the projected area; and a correction coefficient calculation unit configured to, based on the reference points of the projected area, define a projection target area relative to the projected area and to calculate correction coefficients that correspond to the plurality of projection units, the image as the whole corrected based on the correction coefficients being projected to the projection target area from the plurality of projection units.