A thrust converter comprising a coupling pipe (30) outputting reciprocating motion of a first rotary reciprocation converting means (115) and a coupling nut (28) interposed between the first rotary reciprocation converting means (115) and the coupling pipe (30) while being coupled with the first rotary reciprocation converting means (115) and screw coupled with the coupling pipe (30) wherein the screw coupling position of the coupling nut (28) and the coupling pipe (30) is set arbitrarily by driving the coupling pipe (30) through a second servo motor (31); Axial length can be shortened for a required stroke and motor control is simplified because synchronous operation of two motors is not required at the time of feeding to a work grasping position;