A metallic core for a rubber track includes wing portions to be embedded in an elastic member of the rubber track, an engaging portion at the center of the core for engaging a driving sprocket, and a pair of protrusions provided on both sides of the engaging portion and projecting from the inner circumferential surface of the rubber track. Each of the protrusions is provided with at least one extension extending in the longitudinal direction of the rubber track, and the top surfaces at the extension and at the center of the protrusion for rolling an idler thereon are connected to each other with a surface gently inclined downward toward the center of the protrusion, thereby maintaining idlers rolling on the metallic cores of a rubber track at a substantially constant level without upward and downward movements. The protrusions may be formed with shoulders along their longitudinal directions and in symmetry with each other to form upper top surfaces which are flat for an upper idler and lower top surfaces for a lower idler, the lower top surfaces at the extensions are higher than those at the centers of the protrusions.