To prevent the deterioration of seat holding ability and safety by preventing a housing box from falling out or damaging.

This device is constituted to comprise a lifting mechanism 20, which is provided with a rocking member 30 for moving a seat cushion in the vertical direction, a screw member 4, one end of which is locked to the rocking member 30 and can freely move in the axial direction, and a housing box 15 provided with a reduction gear mechanism 5, which reduces the rotation of a motor 13 and moves the screw member 4 in the axial direction. In this case, this device is provided with a holding member 14, which is locked to a fixing section 6 for locking and holding the housing box 15.