A method of manufacturing a tire reinforcing material for achieving uniformity in stiffness at a joint of a ply of a cord fabric and improving dynamic uniformity of a tire shaped and vulcanized, the cord fabric constructed of warp cords arranged in parallel with each other and wefts arranged substantially orthogonally to the warp cords, includes: a topping step for coating with rubber the cord fabric having the wefts folded back at respective warp cords at opposing ends of the cord fabric to produce a ply; a cutting step for cutting the ply into ply pieces each having a predetermined length; a joint forming step for removing one to five cords at each of opposing ends of the ply to form a joint; and a joining step for joining together the joint of one of the ply pieces and the joint of another ply piece.