A machine tool has a pair of guide rails and a pair of upper and lower ball screws. No forces are transferred between a beam of a double column frame and a movable unit. The guide rails are spaced from each other and disposed on a bed, and the movable unit is horizontally movably supported on the guide rails. The upper and lower ball screws are mounted respectively on upper and lower portions of the double column frame for moving the movable unit horizontally. Even when the beam is deformed by flexing downwardly by gravity, the movable unit moves stably and smoothly at a high speed to machine a workpiece highly accurately without being adversely affected by the deformation of the beam.