An irradiation field limiting device includes a plurality of aperture leaves arranged in a thickness direction, a flexible linear member secured to a thick portion of the aperture leaf, a driver section which drives the linear member a specific amount, and the like. The first cable may have a right hand threads on its core, while the second cable may have left hand threads along its core. A drive lever pivoted at a fixed drive axis on the housing has an outer end pivoted at an outer drive axis on the outer end of the actuating arm of the main lever.