To improve a stop accuracy by using a scissors gear with a less backlash to an intermediate transmission gear meshed a drive meshed tooth on a motor output shaft with a driven spur gear stuck to a screw shaft, in the rotation drive device of the screw shaft screwed with the straight line opera tion rod of a straight line operation machine.

A motor M is driven and a screw shaft 6 is rotated through a drive meshed tooth 8, intermediate transmission gear 10 and driven spur gear on the output shaft and a nut member 3 screwed to this screw shaft 6 is advanced/retreated in a straight line direction by the spline engagement with the inner periphery surface of a cylindrical housing 4 and a straight line operation rod 2 is moved in a straight line. A scissors gear is used to the intermediate transmission gear 10 in such straight line operation machine. This scissors gear, consisting of first/second gears separated in an axial direction, is meshed to the drive meshed tooth 8 and driven spur gear 7 in such state that a backlash is not generated by slightly shifting the phase of the periphery direction of respective meshed teeth of the first/second gears. Thereby, the stop accuracy of an operated body is improved.