Abstract: A supporting element guided in the back and forth direction by the inner circumferential face of a guide hole of a housing is made of a pair of half pieces. The back face of the rack is supported by the front ends of the half pieces. Both half pieces are pressed to the back of the rack and to the inner circumferential face of the guide hole. The inner circumferential face of the guide hole is arranged on a first fictitious cylindrical face having an axis along the back and forth direction. The outer circumferential face of each of the half pieces has a small diameter portion and a great diameter portion. The small diameter portion is arranged on a second fictitious cylindrical face, which has an axis along the back and forth direction and the diameter of which is smaller than that of the first fictitious cylindrical face.