An object of this invention is to enable high-speed and high-pressure locking of a mold with a small-capacity electric motor by employing a nut-screw system in a locking device of, for example, an injection molding machine to thereby attain the size reduction of the machine as a whole. The feature resides in the provision of nut-screw system which consists of two screws arranged in series on the rotary shaft and two nuts that engage therewith, one of said nuts being secured to a stationary frame or to a movable platen and the other nut being mounted via a clutch. When the clutch is tightly coupled to the stationary frame or to the movable platen while the shaft is rotating, the two nuts and the rotary shaft undergo a relative movement by an amount corresponding to the pitches of the two screws and thus enable the mold to be locked at a high speed. When the clutch is disengaged, the rotary shaft and only the fixed nut undergo a relative movement, enabling the mold to be locked at a high pressure.