A mold oscillator includes a ball screw unit that has a ball screw 32 having a threaded shaft 29 and a ball nut 30, and has a nut-side shaft 31 fixed to the ball nut 30 coaxially with the threaded shaft 29, wherein the ball screw unit is positioned so that the central axis of the ball screw unit extends in the vertical direction. A casing surrounds at least part of the ball screw 32. The ball nut 30 of the ball screw 32 and part of the threaded shaft 29 of the ball screw 32, on which part the ball nut 30 is engaged with the threaded shaft 29, are submerged in a lubricating oil so as to be able to lubricate the ball screw 32 with an oil bath.