A housing is held by a bracket of a vehicle seat and receives a worm wheel and a threaded shaft. The shaft is driven linearly through the worm wheel. One end of the housing contacts the bracket and includes a receiving recess, which is recessed in the axial direction of the worm wheel. A sensor unit is installed in the recess of the one end of the housing and includes a support wall, which supports the housing relative to the bracket in the axial direction of the worm wheel.