The technique of the present invention adjustably sets an image area on a center of an image display area of an image display device for displaying an image expressed by an input image signal and a mask display area surrounding the image area for displaying a black image, based on a user's setting of a parameter representing a projection size. The technique contracts or expands the image expressed by the input image signal to make a size of the image expressed by the input image signal fit a size of the image area in the process of generating a display image signal from the input image signal, while generating the display image signal to make the black image displayed in the mask display area. The projection size is thus adjustable according to the size of the image area and the size of the mask display area, which are set based on the parameter representing the projection size. This arrangement ensures simple adjustment of the projection size of the image by the projector.