PURPOSE: To achieve a high efficiency in directional adjustment of incident X rays, by mounting two metal fine wires each on both incoming and outgoing sides of the X rays along the diameter of a chi-ring.

CONSTITUTION: Metal fine wires 2 and 3 are mounted on the upstream and downstream sides of X rays in a chi-ring 1 with mounting devices 4 having a fine position adjusting function and a fine tension adjusting function. The fine wires 2 and 3 are so mounted as to pass through a rotating shaft of the ring 1 and consequently, the intersection of the fine wires 2 and 3 makes a point on the rotating shaft of the ring 1. Under such a condition, X rays are incident from one side of the ring 1 to check an incident state thereof downstream by photography. A positional difference between the two fine wires the one on the upstream side and the other on the downstream side in a photograph causes a vertical deviation in the incident X rays and the deviation of the incident X rays from the rotating shaft of the ring 1 can be learned by taking into account of a distance between the fine wire 2 upstream and the fine wire 3 downstream. This makes a point to be corrected in optical systems clear thereby enabling incident-wise centering of the X rays efficiently.