1. A projection image position adjustment method of applying position adjustment to two projection images using two adjustment images, the two projection images being projected on a projection surface to have an overlay area by two of a plurality of projectors in a multi-projection display, the method comprising: providing, to the two projectors, two adjustment image data respectively corresponding to the two adjustment images, each showing a pattern of specific characteristics in the overlay area when the two adjustment images are projected with an appropriate positional relationship; calculating an evaluation value correlated to the characteristics based on image capture data that is derived by capturing an image of the projection surface with the two adjustment images projected thereon by the two projectors; and subjecting the two projection images to the position adjustment based on the evaluation value.