Abstract: An anti-backlash nut mechanism includes two segmental nuts composing a nut which are not separated when the nuts are disengaged from a male screw. An anti-backlash nut has a main nut and a trailing nut which can be engaged with a common male screw, a coil spring for biasing the main and trailing nuts away from each other, a cylindrical portion formed in conjunction with the main nut so as to surround the lead screw, an insert portion formed in conjunction with the trailing nut to be inserted into the cylindrical portion, pins projecting in the radial direction from either one of the cylindrical portion and the insert portion, and grooves formed around the other one of the cylindrical portion and the insert portion so as to receive the pins from the edge of the cylindrical portion and to engage with the pins in the turning direction of the nuts.