An image processor outputs an image containing relatively much information of a central area in the horizontal direction. A mask composed of an upper mask and a lower mask is superimposed on the image. The upper mask is concave on the lower side thereof and disposed along the upper side of the image. The lower mask is concave on the upper side thereof and disposed along the lower side of the image. When the image is masked with this mask, the peripheral portion of the image that is prone to be distorted becomes invisible. Therefore, discomfort a user feels because of distortion can be reduced. In addition, the user can view the image with the much information of the central area of the image in the horizontal direction.