PURPOSE: To prevent chipped block or crack from developing in rib at molding with two splits molds by a method wherein kerf forming blades are provided at the zigzag recessed pats of a main groove forming rib, which is provided near the middle of a tread width.

CONSTITUTION: A mold K consists of two split molds, which are bisected to the peripheral direction of a tire and the parting position X-X" of which lies in the position slightly upward from the middle CL of the tread width of a tread forming face and which has main groove forming ribs 13 extending to the peripheral direction of the tire in zigzag fashion. Kerf forming blades 15 are provided at the zigzag recessed parts of the main groove forming rib 13, which is provided near the middle of the tread width, among a plurality of the main groove forming ribs, which are disposed in row. The zigzag projected parts of a rib, which are molded corresponding to the zigzag recessed parts of the main groove forming rib, bisect and weaken the frictional force developing at the releasing of the tire from the mold, resulting in prevent the broken block and crack from developing at the zigzag projected parts of the rib. The kerf forming blade 15 must be provided without fail at the zigzag recessed parts of the main groove forming rib 13. Even if the blade is provided on the zigzag projected part, no preventing effect is obtained against broken block and crack.