PURPOSE: To conduct an accurate positioning with refefence to a screw portion of a bolt inseted into a hole formed to a predetermined size in a part having no finished reference surface.

CONSTITUTION: Parts 9 which have bolts inserted therein and which are fed successively are positioned by a positioning means 13 and then held by a chuck means 14. A spindle 17 having at one end portion thereof a socket which is engaged with the bolts, and at other end portion thereof a polygonal boss is provided such that the spindle 17 is slidable in the axial direction. The spindle 17 and chuck means 14 are rotated at a predetermined angle by a rotating means to change the postures thereof, and transferred by a transfer means in the perpendicular direction with respect to a sub-transfer line side and in the perpendicular direction with respect to a mounting surface of a front part body. The spindle 17 is further rotated by a nut runner means 19 moved to and fro in a mounting position on the axial line of the bolts, and the axial movements of the spindle 17 are sensed by a detector