To obtain a long stroke, while simplifying structure.

Three ball screws 2, 3 and 4 are arranged on the same shaft. A shaft 32(42) of one ball screw 3(4) of adjacent ball screws 2 and 3(3 and 4) is provided with a hollow part 32a (42a) capable of housing a shaft 22(32) of the other ball screw 2(3) like an insert die. An end of the shaft 32(42) of the one ball screw 3(4) and a nut 21(31) of the other ball screw 2(3) are fixed to each other, and each of the three ball screws 2, 3 and 4 is provided with a bush 12 and a damper 13 for stopping rotation in the condition wherein the shaft 22(32)is housed in the hollow part 32a(42a) to continuously extend and shrink and in the condition wherein the shaft 22(32) is extended from the hollow part 32a(42a).