To prevent the occurrence and the growth of the crack of an elastic material to cover a guide projection while preventing the biting-in of stones by the guide projection.

To a core main body 2 long in the lateral direction, an engaging part 3 for driving wheel on the way in the lateral direction; a guide projection 6 having a top part erected at the upper surface side and projecting long at least to one side in the longitudinal direction, at least at one side at the left side and the right side of the engaging part 3, and lateral regulation projections 27 and 28 projecting from the front side and the rear side of the core main body 2, are formed. The guide projection 6 is overlapped with the lateral regulation projections 27 and 28 in the lateral direction, and a fin part 9 is formed at the front and the rear side of the erecting part of the projection 6, and at the lower side of the projecting side of the top part, while a groove 10 in the longitudinal direction is formed between the lover edge of the fin part 9, and the lateral regulation projections 27 and 28.