1. An image processing method, comprising: detecting a distorted image before correction projected on a screen by a projection-type image display device; detecting a position and a size of a distorted quadrangle projected on the screen; searching for a same shape as an input image having a maximum area inside the distorted image; searching for a rectangle having the maximum area with a same aspect ratio as the input image inside the distorted quadrangle; searching for a center position from which a minimum value of a distance, in a direction of a diagonal line to a side of the distorted quadrangle, is maximum; determining a maximum rectangle with the same aspect ratio inscribed in the distorted quadrangle with the center position, as the rectangle having the maximum area; and correcting the input image in a manner that the distorted image becomes the searched-for shape having the maximum area.