1. A method for determining a projection area of an image, comprising: an input step of inputting an image sequence having a plurality of images; a detecting step of detecting locations of projection areas of the respective of images in the image sequence; a relationship classification judging step of judging a relationship classification between an image being projected and a previous image before the image being projected based on a relationship between the location of the projection area of the image and the location of the projection area of the previous image; and a determining step of determining the locations of the projection areas of the respective images based on the relationship classification judged in the relationship classification judging step.