1. A projection system comprising: a plurality of projectors to project images to form an image as a whole on a projected area which is a screen area on which a content image is projected without a frame; circuitry configured to perform: outputting images for calibration to projectors that are in charge of areas of corners of the projected area among the plurality of projectors, the images for calibration including positioning markers that are indications for positioning to the corners of the projected area; detecting, 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, the reference points being different from the positioning markers that are indications for positioning to the corners of the projected area; and calculating correction coefficients, based on the reference points of the projected area, and defining a projection target area which corresponds to the screen area, the correction coefficients corresponding to the plurality of projectors, the image as the whole corrected based on the correction coefficients being projected to the projection target area from the plurality of projectors, wherein the reference points are detected by detecting a first set of points in line with at least one of the position markers, and detecting a second set of points, corresponding to points of the first set of points, the second set of points displaced outwardly on the projected area relative to the first set of points, the second set of points corresponding to the reference points.