is provided between the ends of the passages 22 a 22 b and 22 c of the extrusion head 18 and the die plate 20 as shown in fig1 a 1 b 5 6 a 6 b and 6 c the passage forming die 32 includes at a central portion thereof a passage 34 b through which the unvulcanized rubber b flows and also includes a passage 34 a through which the unvulcanized rubber a flows at a side in the direction indicated by arrow l and a passage 34 c through which the unvulcanized rubber c flows at a side in the direction indicated by arrow r the passage 34 a and the passage 34 b are connected to each other the passage 34 a has a stepped portion 36 at a side in the direction indicated by arrow u a first weir 38 which serves as a flow dividing mechanism is provided at an end of the stepped portion 36 in the direction indicated by arrow d that is a portion at which the passage 34 a and the passage 34 b are connected so as to protrude toward the passage 34 b in the direction indicated by arrow d as shown in fig1 a the dimension of the first weir 38 protruding toward the passage 34 b is gradually made shorter from a substantially intermediate portion thereof in the direction indicated by arrow l an end 38 a of the first weir 38 in the direction indicated by arrow d is disposed at a substantially intermediate position with respect to an.