a cylindrical rod holder;

a rod inserted into the rod holder and including at its front end a nut holding part insertable into the hole of a nut, said rod being constructed so as to be driven into forward and backward movement by an actuator;

a nut stopper, provided at the front end of the rod holder, for supporting the nut coaxially with the rod and for permitting the forward feed of the nut when the nut holding part is inserted into the nut hole and subsequently pushes forward the nut stopper by the forward movement of the rod;

a piston provided on the rod to slide on an inner periphery of the rod holder; and

a seal for airtightly inserting a part of the rod located forward of the piston into the rod holder, said seal being provided on the inner periphery of the front end of the rod holder,

wherein in the rod holder, an air chamber is formed which is defined at the front and back ends by the seal and the piston, respectively, and changes the air chamber's volume in accordance with the forward/backward movement of the rod,

the rod is formed with an air outlet provided at the front end section of the nut holding part to blow air to the nut carried on the nut holding part against dropping off of the nut from the nut holding part, an air inlet provided backward of the nut holding part to connect with the air chamber, and an air passage connecting the air outlet and the air inlet, and

the forward movement of the rod allows the air in the air chamber to be pressurized by the piston to blow the pressurized air to the nut carried on the nut holding part, and then allows the nut to be fed as being held on the nut holding part by the air blown to the nut.