1. An apparatus for detecting an edge center of an object comprising: camera means for taking a picture of the object and generating an analog image signal which represents a brightness of an image on individual picture elements, wherein the individual picture elements form a two dimensional matrix with an x dimension and a y dimension; an A/D converter for converting the analog image signal to digital image data according to a gray scale; memory means for storing at least the digital image data; means for calculating differential data which corresponds to the digital image data between each of the individual picture elements which are adjacent to each other in a direction of the x dimension; means for converting the differential data corresponding to each of the individual picture elements that are located in a selected region within an edge zone defined in the direction of the x dimension into an index value for each of a plurality of indices such as small, medium and high; means for converting each index value for each of the individual picture elements that are located in the selected region into at least one edge value and calculating at least one preliminary resultant center position from at least one of the edge values; and means for weighing an x position of the x dimension of each of the individual picture elements in the selected region based on at least one of the preliminary resultant center positions and calculating a subsequent center position based on the weighted x positions of the individual picture elements in the selected region, wherein the subsequent center position is the edge center of the object in the x dimension.