A belt supporting apparatus includes a bottom portion, first and second wall portions integrally formed with the bottom portion so as to be parallel to each other, a holding portion integrally formed at the second wall portion and an opening formed at the bottom portion opposes the holding portion, wherein the housing houses a toothed belt so as to be slidably in such a way that a toothed portion of the toothed belt is facing the first wall portion, and a back surface formed at the other surface of the toothed belt is facing the second wall portion, and the toothed belt is prevented from being moved out of the housing in the vehicle height direction by means of the holding portion.