A vibration testing system which can perform vibration test by vibrating a work with a long period and a large acceleration amplitude. A direct drive converter in the vibration testing system comprises a direct drive converter frame fixed to the frame of the vibration testing system, an input shaft pivotally supported rotatably for the direct drive converter frame and coupled with the rotating shaft of a servo motor, a square thread formed at least partially on the outer circumferential surface of the input shaft, a roller having a tubular surface abutting against the flank of the square thread, a roller unit having a rotating shaft embedded therein for pivotally supporting the roller rotatably through a cylindrical roller bearing which is substantially entirely contained in the groove ofthe square thread, a rail fixed to the direct drive converter frame and sliding the roller unit straight along the axial direction of the square thread, and an output shaft coupled with the roller unit directly or indirectly and supporting a movable table at the upper end thereof wherein the roller screwing together with the square thread moves along the groove of the square thread as the input shaft rotates, and the output shaft interlocked with straight movement of the roller unit along the rail also moves straight to move the movable table up and down.