An automotive power seat sliding device comprises first and second lower rails secured to a vehicle floor; first and second upper rails slidably and respectively engaged with the first and second lower rails and supporting thereon a seat; a cross beam member having both ends secured to the first and second upper rails respectively, so that the first and second upper rails slide on and along the first and second lower rails like a single unit; a drive shaft extending along the first upper rail and having a spiral groove formed therearound; a nut member secured to the first lower rail and operatively engaged with the drive shaft; an electric power unit supported by the cross beam member for rotating the drive shaft about an axis thereof; a supporting member connected to the first upper rail for rotatably supporting one end portion of the drive shaft; and a protector bracket secured to the first upper rail and having a recessed structure into which the other end of the drive shaft is received without contacting the recessed structure.