PURPOSE: To cope with many output modes by forward/reversely rotating another member of screw or nut members through a speed control means formed of a first/second fluid pump/motors and pipes for connecting them

CONSTITUTION: A speed control means 40 is formed of a first fluid pump/motor 41 of predetermnined capacity associated with a prime mover 22, second fluid pump/motor 42, associated with a nut member 12, of equal capacity to the first fluid pump/motor 41 and pipes 43, 44 of forming a fluid circulating circuit with both the fluid pump/motors 41, 42, and control valves 44, 45, having a 4-port 2-position selector valve 45 and a 4-port 3-position selector valve 46 mounted between pipes 43, 44, control a flow of fluid between the first fluid pump/motor 41, associated with the prime mover 22, and the second fluid pump/ motor 42, associated with the nut member 12, to cause a feed speed of a screw member 11 to be variably controlled by relatively rotating the nut member 12 in an equal or reverse direction to the screw member 11.