Patent Number: 
Section: claims

1. A radiation therapy apparatus comprising:a multi-leaf collimator device including a pair of collimator components on a same plane, each collimator component including a plurality of leaves, one collimator component of the pair of collimator components and an other collimator component of the pair being arranged close to one another such that the collimator components face one another across an irradiation axis and set a desired irradiation field by individually moving the leaves,wherein leaves of the one collimator component are arranged with an offset with respect to leaves of the other collimator component along a direction in which the leaves of the one collimator component are arranged, within a range of a leaf-width. 2. A radiation therapy apparatus according to claim 1, wherein the leaves are movable individually along an arc-shaped path around a radiation source as a center. 3. A radiation therapy apparatus according to claim 2, wherein the leaves are arranged orthogonally to a moving direction of the leaves along the arc-shaped path. 4. A radiation therapy apparatus according to claim 1, wherein the offset with which the collimator components are arranged is approximately half the leaf-width. 5. A radiation therapy apparatus, comprising:a multi-leaf collimator device including a pair of collimator components on a same plane, each collimator component including a plurality of leaves, one collimator component of the pair of collimator components and an other collimator component of the pair being arranged close to one another such that the collimator components face one another across an irradiation axis and set a desired irradiation field by individually moving the leaves,wherein the one collimator component is movable with respect to the other collimator component along a direction in which the leaves of the one collimator component are arranged. 6. A radiation therapy apparatus according to claim 5, wherein the leaves are movable individually along an arc-shaped path around a radiation source as a center. 7. A radiation therapy apparatus according to claim 6, wherein the leaves are movable and arranged orthogonally to a moving direction of the leaves along the arc-shaped path. 8. A radiation therapy apparatus according to claim 5, wherein at least one of the collimator components is movable in a range of approximately half the leaf-width.