PURPOSE: To perform easily and quickly an exchanging operation and to permit easy positioning without the possibility of dislodging by providing the 1st screw part, 2nd screw part and 3rd screw part and attaching and detaching a collimator by rotating a nut.

CONSTITUTION: When a nut driver 13 is rotated in a right-hand screw direction, the 2nd screw part 10a of a nut 10 rotates while screwing to the 1st screw part 10b of a stay 1A and therefore the nut 10 advanced downward and the bottom end face thereof contacts with the top end of the screw part 15b of a bolt 15. When the rotating operation of the driver 13 is continued, the nut 10 advances further downward and is pushed into an embedding hole 14. The 3rd screw part 11 is meshed with the screw part 15b where the downward advance of the nut 10 and bolt 15 progresses to some extent. The bolt 15 is at the same time energized upward by a coil spring 16 and only the nut 10 advances downward. As a result, the 2nd screw part 10a is detached from the 1st screw part 10b of the stay 1A and the 3rd screw part 11a is meshed with the screw part 15b of the bolt 15.