Abstract: Periphery detectors are arranged being juxtaposed closely, along the longitudinal direction of the log which is to be rotated one revolution around a virtual center, thereby a plurality of peripheries of cross sections along the longitudinal direction are detected for the log. The direction of the largest right circular cylinder is determined according to the largest inscribed circles for the peripheries of three cross sections (i.e., the ones adjacent to both the ends of the log and the one in the middle of the log) and the largest right circular cylinder itself is determined according to the peripheries of all cross sections detected. Thus the center line of said largest right circular cylinder is adopted as the turning center. The positions of a pair of inclined beams or swing arms are adjusted for canceling the deviation between the turning center and the virtual center.