Abstract: Detection of tilt angle of an optical axis of a camera in relation to a vertical line of an image display surface by means of information from image processing, without relying on specific optical apparatus, sensors or the like. A localized focus degree is determined for a plurality of regions within the image display surface and the tilt angle is determined according to an amount of deviation of such focus degree. As values showing such localized focus degree, measurement of image brightness is performed along an extension of such region in an abscissa direction, throughout the whole region in the ordinate direction. Only a difference in brightness between adjacent regions that exceed a designated amount is accumulated, and such accumulated value is divided by a value of an area of a designated region. Such divided value is a focus value in the abscissa direction. Similar procedure is done for the ordinate direction.