To solve the problems that in the conventional bicycle of a diamond type structure frame or its varied structure frame, the whole is complicated and large, and heavy, and that in the case of a collapsible bicycle of this structure frame, folding and assembling work is complicated, and it is impossible to carry the folded bicycle while rolling.

A back fork connected by fixing a sheath post gripped by caulking a seat pillar supporting a seat of a saddle and a rear wheel axle is one side of a single triangular frame. A front shaft connected by fixing a sheath post of the saddle and a handle sheath of a front fork is another side of the single triangular frame. A horizontal shaft connected by fixing the rear wheel axle and the handle sheath of the front fork is the remaining side of the single triangular frame.