show a close value to arctangent of expansion ratio therefore it is realized that expansion ratio ro ri may be set within a scope of 2 u2266 ro ri u2266 8 as a guide for efficiently orienting short fiber in y direction rubber mixed with respective short fiber of aspect ratio 2 to 5 370 500 is extruded from the expansion die 1 attached with the center block 7 ro ri 7 5 to mold the cylindrical body including short fiber in the cylindrical body including short fiber of aspect ratio 370 and 500 is molded then in the cylindrical body including short fiber of aspect ratio of 370 and 500 short fiber is in the state of lump and it is not proper as a rubber material that means if short fiber exists in the state of lump in the rubber material deformation occurs during vulcanization and strength distribution becomes uneven and examined orientation rate of the vulcanized rubber including short fiber that is obtained by vulcanizing the cylindrical body including short fiber having aspect ratio 2 to 5 y direction is 36 2 x direction is 37 1 and z direction is 26 7 and the orientation rate is higher in x direction than in y direction the results of this test 3 are shown in table 3 based on the results of the test if short fiber aspect ratio is out of.