PURPOSE: To have adequate vulcanization speed and achieve high hardness and low dynamic magnification at low cost and with ease by mixing the predetermined staple for raw material rubber which consists of natural rubber or synthetic rubber or their mixture.

CONSTITUTION: Staple which has the length of fiber, L, of 10mm or less, the ratio of the length of fiber, L, to diameter of fiber, D, L/D, of 40 or more, and initial elastic modulus of 40g/d or more is mixed for raw material rubber which consists of natural rubber or synthetic rubber or their mixture. If the above constitution is realized, staple which has considerably high elastic modulus when compared with rubber itself is mixed in raw material rubber. Therefore, movement of rubber is restricted, and high elastic modulus and low dynamic magnification are realized. Also, since staple absorbs external energy, dynamic loss is increased and vibrationproof characteristics are improved due to mixture of staple. Conventional defects that vulcanization speed is accelerated, machinability is worsened, heat resistant agening property is lowered, and durability is worsened do not occur.