PURPOSE: To facilitate opening/closing of a hinge joint and folding of a pipe by forming a pair of seat parts in butting ends of respective pipes, constructing a hinge joint, by which one side edge of each seat part is freely opened/closed by means of a hinge shaft, and journalling a rod, in which a hook is formed, to an operation lever.

CONSTITUTION: A main pipe in a bicycle frame is constructed in such a way as butting a front side pipe 11 fixed on a head pipe side and a rear side pipe 12 fixed on a seat tube 3 side into each other. In butting ends of the respective pipes 11, 12, a pair of seat parts 111, 112 are formed, and a hinge joint 11, by which one side edge of each seat part is freely opened/closed, is constructed. On the other hand, an engaging part 13 is formed on the other side edge of the seat part 111 in one pipe end, while an operation lever 17 is journalled freely rotationally to the other side edge of the seat part 112 in the other pipe end. A rod 18, in which a hook 20 to be engaged with the engaging part 13 is formed at the tip, is journalled in the operation lever 17 freely rotationally. The hook 20 is energized in the direction separating from an engaging groove by means of a spring 21.