Collimator and radiation irradiator

A radiation irradiator having a radiation source and a collimator for applying a radiation from the radiation source to an object through an aperture, the collimator comprising:

• a pair of first plate members having a shielding property against a radiation and movable in a direction parallel to surfaces thereof, the pair of first plate members defining a radiation passing aperture by a spacing between respective opposed end faces;
• a pair of second plate members having a shielding property against a radiation, parallel to the pair of first plate members, and movable in a direction parallel to surfaces thereof, the pair of second plate members having end faces opposed to each other, the pair of second plate members overlapping the pair of first plate members at least partially so as to block any other radiation than the radiation passing through the aperture;
• a pair of third plate members having a shielding property against a radiation and parallel to the pair of second plate members, the pair of third plate members having respective end faces opposed to each other with a predetermined spacing, the pair of third plate members overlapping the pair of second plate members at least partially so as to block any other radiation than the radiation passing through the aperture;
• an adjusting mechanism which adjusts the aperture by moving the pair of first plate members;
• and
• a follow-up mechanism which causes the pair of second plate members to move following the pair of first plate members with movement of the first plate members.