According to an aspect of the present invention, a drive mechanism for a power seat apparatus for a vehicle includes an electric motor, a speed-reduction gear mechanism, a gear housing for housing therein the speed-reduction gear mechanism and supporting thereto the electric motor, the gear housing including a housing body having an accommodating space, a cover mounted to the housing body, and a retaining member provided on the housing body for fixedly retaining the cover on the housing body by rotating the cover relative to the housing body upon assembly, and a bracket through which the gear housing is supported by the driven member, the bracket including a receiving portion and a rotation-restricting surface.