To provide high flexibility in designing a bicycle frame to minimize a drag according to optimum aerodynamic consideration by installing only one rear fork including right and left branches and preliminarily molding and hardening both branches prior to the manufacture of a whole frame.

Each rear fork includes a left branch 21 and a right branch 2 having a symmetric positional relationship with each other. Also, the left branch 21 and the right branch 22 of a rear frame are respectively joined to a wheel assembly via a pair of dropouts 26. Furthermore, the left branch 21 and the right branch 22 are made of a plurality of prepreg layers capped to a single mandrel. After the removal of the mandrel, the branches 21 and 22 before cured are placed in a mother die. This mother die is made of an upper die member 31 and a lower die member 32. Furthermore, an expanded bladder is placed within the un-cured branches 21 and 22, and the mother die is closed. Thereafter, pressure is applied at a thermal hardening process. The right and left branches 22 and 21, after cured, and bonded to each other by use of a suitable adhesive.