To make a screw smoothly movable in the axial line direction when an electric motor becomes not operable and can not rotate from a nut side, in an electrically-driven actuator which rotates a nut on the basis of the operation of the electric motor and moves the screw combined with the nut in the axial line direction.

An electrically-driven actuator 1 is equipped with a nut 11 rotating by the operation of an ultrasonic motor 6, a screw 21 for converting the rotating motion of the nut 11 to a rectilinear motion in the axial line direction, a regulating member 22 which allows or regulates the rotation of the screw 21, and an electromagnetic clutch 26 which allows or regulates the rotation of the regulating member 22. A piston head 29c for pressuring a member 31 to be pressurized by the rectilinear motion of the screw 21 is so linked with the screw 21 by a thrust bearing 29b that relative rotation is enabled.