Provided is an aperture-plate moving mechanism, including: a drive block 308 fixed to an aperture plate 301; a linear motion guide 306 for allowing the drive block to move along an axis while preventing the drive block from moving in other directions; a feed screw 302 laid in a direction of the axis; a nut member 305 having a threaded hole engaged with the feed screw, the nut member being prevented from rotating due to a rotation of the feed screw; and an urging member 309 for pressing the drive block onto the nut member in the direction of the axis. With respect to the direction of the axis, the contact portion of either the drive block or the nut member is a convex surface, while the contact portion of the other member is a concave surface having a larger radius of curvature than the convex surface.