A radiation diaphragm apparatus, which is adapted to form a radiation field, which is an exposed area of an object to be examined by radiation from a radiation source, comprises a plurality of diaphragm elements which are closely arranged in a first direction and movable along a second direction substantially normal to the first direction and each of which has a hole of a given shape formed to penetrate through it in the first direction, a shaft penetrating through the hole of each of the diaphragm elements, and a wear-resistant surface member coating the shaft. The shaft supports each of the diaphragm elements at a point of contact with the periphery of the hole. The diaphragm elements move along the second direction with support by the shaft.