To dispense with slow running at switching from speed control to power control.

A liquid holder 80, which holds liquid and also expands and contracts in the direction of rectilinear motion, interlocking with the compression and expansion of the liquid, is introduced into the rear stage of a conversion mechanism 5 which converts rotational motion into a rectilinear motion, when it transmits the output of an electric motor 3 to an actuator 7. In addition, the controller 10 performs both the speed control, which is performed by feeding back a detection result Vf of a speed detection means 3b, and the power control, which is performed by feeding back a detection result Ff of a power detection means 60 by the control of the electric motor 3, and also performs changeover without slowing down, when the user switches the speed control over to the power control during its control.