An aperture comprises blocking members and for blocking X-rays. The blocking member has: an edge formed on an arc A1 lying on a circumference CF2 of a circle with radius r2 around a focal spot f in an XY-plane; an edge connected to one endpoint of the edge, the edge being formed to lie on the side of the X-ray detector with respect to an arc A11 contiguous to the arc A1; and an edge connected to the other endpoint of the edge, the edge being formed to lie on the side of the X-ray detector with respect to an arc A12 contiguous to the arc A1. The blocking member has: an edge formed on an arc A2 lying on the circumference CF2 of the circle with radius r2 around the focal spot f in the XY-plane; an edge connected to one endpoint of the edge, the edge being formed to lie on the side of the X-ray detector with respect to an arc A21 contiguous to the arc A2; and an edge connected to the other endpoint 23b of the edge, the edge being formed to lie on the side of the X-ray detector with respect to an arc A22 contiguous to the arc A2.