A downhole actuator device comprises a central shaft formed with external threads; a sleeve formed with internal threads and placed substantially concentrically around the shaft; and an electromotor arranged to rotate at least a first one of said shaft and said sleeve so that a second one of said shaft and said sleeve may be moved linearly relative to the first one of said shaft and said sleeve. The actuator further comprises a plurality of roller screws that are rotatingly supported between the shall and sleeve in engagement with said external threads on the shaft and said internal threads on the sleeve.