A geometrical correcting method allowing geometrical correction to be performed simply, accurately and in short a time, even in a multi-projection system including a screen of complex shape and projectors complexly arranged, for significantly improving the maintenance efficiency. Test pattern images having feature points are projected by respective projectors, and captured and displayed on a monitor, approximate positions of the feature points are designated and input while referring to the displayed test pattern captured image, and the accurate positions of the feature points in the test pattern images are detected according to the approximate position information. An image correction data for aligning the images projected by the projectors is calculated from the detected positions based on the feature points, the coordinate information of the feature points in a predetermined test pattern image, and the coordinate position relationship between a separately predetermined contents image and the test pattern captured image.